Current versions of MediaWiki have the front page accessible via
http://www.example.com/wiki/index.php/Main_Page
Instead of the older version of
http://www.example.com/wiki/index.php?title=Main_Page
That second URL is literally how the PHP script would be called in most casual web apps, but the above URL is cleaner and more desirable. Now, I can see how that could easily be converted with an Apache mod_rewrite rule:
RewriteRule ^/index.php/(.*)$ /index.php?title=$1
But there's no htaccess file in the default MediaWiki setup. So how are they doing the redirect?