Catch all requests for www.domain.tld and redirect to domain.tld

$HTTP["host"] =~ "^www\.(.*)" {
   url.redirect = ( "^/(.*)" => "http://%1/$1" )
}

Doing some work on your web site but you don't want other people to see what you're doing. Give them a redirect (like the flickr massage screen).
You need to use your own ip of course.

# redirect all visitors to alternate site but retain full access for you
ErrorDocument 403 http://www.alternate-site.com
Order deny,allow
Deny from all
Allow from 99.88.77.66

// redirect domain.com to www.domain.com (or vice versa)
// helps substantially with delicious links and other SEO optimization
// see for discussion

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

Upgrades woohoo! Tell those lusers to hold their horses while you happily debug. Some .htaccess magic, just sub in your IP:

<IfModule mod_rewrite.c>
        RewriteEngine On
        # 'under construction' override
        RewriteCond %{REMOTE_ADDR} !^127\.0\.0\.1$
        RewriteRule ^(.*)$ maintenance.html
IfModule>

// Redirects a user to the current domain, the current directory, and the new page. Won't break if you move the scripts to another folder/domain etc.

header ("Location: http://" . $_SERVER['HTTP_HOST'] . dirname($_SERVER['PHP_SELF']) . "/newpage.php");

Here is an easy way to redirect one url to another if you don't need any fancy regex pattern matching.

Permanent 301 redirect

Redirect permanent /relative_source_url http://full_destination_url/

-or-

Temporary 302 redirect

Redirect temporary /relative_source_url http://full_destination_url/

To redirect requests for www.example.com to example.com (without the www) put this in your .htaccess:

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

And to do the reverse (redirect non-www to www), try this:

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

Make sure you have mod_redirect in server.modules, then add this to your lighttpd.conf:

$HTTP["host"] =~ "^domain\.com$" {
  url.redirect = (
    "^/(.*)" => "http://www.domain.com/$1",
    ""       => "http://www.domain.com/"
  )
}

I wasn't able to come up with a way to do it in a single rule. It seems that only "" (not even "(.*)") will match the root request.