1 - htaccess snippets
Rewrite and Redirection

Note: It is assumed that you have mod_rewrite installed and enabled.
Force www

RewriteEngine on
RewriteCond %{HTTP_HOST} ^example\.com [NC]
RewriteRule ^(.*)$ http://www.example.com/$1 [L,R=301,NC]

Force www in a Generic Way

RewriteCond %{HTTP_HOST} !^$
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteCond %{HTTPS}s ^on(s)|
RewriteRule ^ http%1://www.%{HTTP_HOST}%{REQUEST_URI} [R=301,L]

This works for any domain. Source
Force non-www

It's actually recommended to remove www from your domain. Surprise surprise!

RewriteEngine on
RewriteCond %{HTTP_HOST} ^www\.example\.com [NC]
RewriteRule ^(.*)$ http://example.com/$1 [L,R=301]

Force HTTPS

RewriteEngine on
RewriteCond %{HTTPS} !on
RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI}

Force Trailing Slash

RewriteCond %{REQUEST_URI} /+[^\.]+$
RewriteRule ^(.+[^/])$ %{REQUEST_URI}/ [R=301,L]

Redirect a Single Page

Redirect 301 /oldpage.html http://www.yoursite.com/newpage.html
Redirect 301 /oldpage2.html http://www.yoursite.com/folder/

Source
Redirect an Entire Site

Redirect 301 / http://newsite.com/

This way does it with links intact. That is www.oldsite.com/some/crazy/link.html will become www.newsite.com/some/crazy/link.html. This is extremely helpful when you are just "moving" a site to a new domain. Source
Security
Deny All Access

Deny from All

But wait, this will lock you out from your content as well! Thus introducing...
Deny All Access Except Yours

Order deny, allow
Deny from All
Allow from xxx.xxx.xxx.xxx

xxx.xxx.xxx.xxx is your IP. If you replace the last three digits with 0/12 for example, this will specify a range of IPs within the same network, thus saving you the trouble to list all allowed IPs separately. Source

Now of course there's a reversed version:
Allow All Access Except Spammers'

Order deny, allow
Allow from All
Deny from xxx.xxx.xxx.xxx
Deny from xxx.xxx.xxx.xxy

Deny Access to Hidden Files and Directories

Hidden files and directories (those whose names start with a dot .) should most, if not all, of the time be secured. For example: .htaccess, .htpasswd, .git, .hg...

RewriteCond %{SCRIPT_FILENAME} -d [OR]
RewriteCond %{SCRIPT_FILENAME} -f
RewriteRule "(^|/)\." - [F]

Alternatively, you can just raise a Not Found error, giving the attacker dude no clue:

RedirectMatch 404 /\..*$

Deny Access to Backup and Source Files

These files may be left by some text/html editors (like Vi/Vim) and pose a great security danger, when anyone can access them.

<FilesMatch "(\.(bak|config|dist|fla|inc|ini|log|psd|sh|sql|swp)|~)$">
    Order allow,deny
    Deny from all
    Satisfy All
</FilesMatch>

Source
Disable Directory Browsing

Options All -Indexes

Disable Image Hotlinking

RewriteEngine on
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^http(s)?://(www\.)?yourdomain.com [NC]
RewriteRule \.(jpg|jpeg|png|gif)$ - [NC,F,L]

Password Protect a Directory

First you need to create a .htpasswd file somewhere in the system:

htpasswd -c /home/fellowship/.htpasswd boromir

Then you can use it for authentication:

AuthType Basic
AuthName "One does not simply"
AuthUserFile /home/fellowship/.htpasswd
Require valid-user

Password Protect a File or Several Files

AuthName "One still does not simply"
AuthType Basic
AuthUserFile /home/fellowship/.htpasswd

<Files "one-ring.o">
Require valid-user
</Files>

<FilesMatch ^((one|two|three)-rings?\.o)$>
Require valid-user
</FilesMatch>

Performance
Compress Text Files

<IfModule mod_deflate.c>

    # Force compression for mangled headers.
    # http://developer.yahoo.com/blogs/ydn/posts/2010/12/pushing-beyond-gzipping
    <IfModule mod_setenvif.c>
        <IfModule mod_headers.c>
            SetEnvIfNoCase ^(Accept-EncodXng|X-cept-Encoding|X{15}|~{15}|-{15})$ ^((gzip|deflate)\s*,?\s*)+|[X~-]{4,13}$ HAVE_Accept-Encoding
            RequestHeader append Accept-Encoding "gzip,deflate" env=HAVE_Accept-Encoding
        </IfModule>
    </IfModule>

    # Compress all output labeled with one of the following MIME-types
    # (for Apache versions below 2.3.7, you don't need to enable `mod_filter`
    #  and can remove the `<IfModule mod_filter.c>` and `</IfModule>` lines
    #  as `AddOutputFilterByType` is still in the core directives).
    <IfModule mod_filter.c>
        AddOutputFilterByType DEFLATE application/atom+xml \
                                      application/javascript \
                                      application/json \
                                      application/rss+xml \
                                      application/vnd.ms-fontobject \
                                      application/x-font-ttf \
                                      application/x-web-app-manifest+json \
                                      application/xhtml+xml \
                                      application/xml \
                                      font/opentype \
                                      image/svg+xml \
                                      image/x-icon \
                                      text/css \
                                      text/html \
                                      text/plain \
                                      text/x-component \
                                      text/xml
    </IfModule>

</IfModule>

Source
Set Expires Headers

Expires headers tell the browser whether they should request a specific file from the server or just grab it from the cache. It is advisable to set static content's expires headers to something far in the future. If you don't control versioning with filename-based cache busting, consider lowering the cache time for resources like CSS and JS to something like 1 week. Source

<IfModule mod_expires.c>
    ExpiresActive on
    ExpiresDefault                                      "access plus 1 month"

  # CSS
    ExpiresByType text/css                              "access plus 1 year"

  # Data interchange
    ExpiresByType application/json                      "access plus 0 seconds"
    ExpiresByType application/xml                       "access plus 0 seconds"
    ExpiresByType text/xml                              "access plus 0 seconds"

  # Favicon (cannot be renamed!)
    ExpiresByType image/x-icon                          "access plus 1 week"

  # HTML components (HTCs)
    ExpiresByType text/x-component                      "access plus 1 month"

  # HTML
    ExpiresByType text/html                             "access plus 0 seconds"

  # JavaScript
    ExpiresByType application/javascript                "access plus 1 year"

  # Manifest files
    ExpiresByType application/x-web-app-manifest+json   "access plus 0 seconds"
    ExpiresByType text/cache-manifest                   "access plus 0 seconds"

  # Media
    ExpiresByType audio/ogg                             "access plus 1 month"
    ExpiresByType image/gif                             "access plus 1 month"
    ExpiresByType image/jpeg                            "access plus 1 month"
    ExpiresByType image/png                             "access plus 1 month"
    ExpiresByType video/mp4                             "access plus 1 month"
    ExpiresByType video/ogg                             "access plus 1 month"
    ExpiresByType video/webm                            "access plus 1 month"

  # Web feeds
    ExpiresByType application/atom+xml                  "access plus 1 hour"
    ExpiresByType application/rss+xml                   "access plus 1 hour"

  # Web fonts
    ExpiresByType application/font-woff                 "access plus 1 month"
    ExpiresByType application/vnd.ms-fontobject         "access plus 1 month"
    ExpiresByType application/x-font-ttf                "access plus 1 month"
    ExpiresByType font/opentype                         "access plus 1 month"
    ExpiresByType image/svg+xml                         "access plus 1 month"
</IfModule>

Turn eTags Off

By removing the ETag header, you disable caches and browsers from being able to validate files, so they are forced to rely on your Cache-Control and Expires header. Source

<IfModule mod_headers.c>
    Header unset ETag
</IfModule>
FileETag None

Miscellaneous
Set PHP Variables

php_value <key> <val>

# For example:
php_value upload_max_filesize 50M
php_value max_execution_time 240

Custom Error Pages

ErrorDocument 400 /errors/breakingbad.html
ErrorDocument 401 /errors/notrespassing.html
ErrorDocument 403 /errors/mordor.html
ErrorDocument 404 /errors/halflife3.html
ErrorDocument 500 /errors/notabugitsafeature.html

Force Downloading

Sometimes you want to force the browser to download some content instead of displaying it. The following snippet will help.

<Files *.md>
    ForceType application/octet-stream
    Header set Content-Disposition attachment
</Files>

Allow Cross-Domain Fonts

CDN-served webfonts might not work in Firefox or IE due to CORS. The following snippet from HTML5Boilerplate should make it happen.

<IfModule mod_headers.c>
    <FilesMatch "\.(eot|otf|ttc|ttf|woff)$">
        Header set Access-Control-Allow-Origin "*"
    </FilesMatch>
</IfModule>

Auto UTF-8 Encode

Your text content should always be UTF-8 encoded, no?

# Use UTF-8 encoding for anything served text/plain or text/html
AddDefaultCharset utf-8

# Force UTF-8 for a number of file formats
AddCharset utf-8 .atom .css .js .json .rss .vtt .xml