[link|http://greasemonkey.mozdev.org/|http://greasemonkey.mozdev.org/]
Now, in addition to user specified css, you can have user specified javascript on a global or per-site basis. It is the next step on your path towards [link|http://wearcam.org/|fully] [link|http://eyetap.org/|mediated] [link|http://www.wearcam.org/historical/|reality].
Already, hackers have produced a [link|http://dunck.us/collab/GreaseMonkeyUserScripts|lot of scripts]. Many of the site specific. Some of them are little more than elaborate killfiles.
As you might expect, this is just the first escalation in the [link|http://www.theregister.co.uk/2005/03/03/google_autolink/|arms] [link|http://diveintomark.org/projects/greasemonkey/___How_To_Disable_Greasemonkey_On_Your_Website.txt|race].