Greasemonkey (homepage here) is a Firefox extension which allows management and application of customised JavaScript to web pages. There is a collection of scripts here: GreaseMonkey Repository
Many of these scripts can be adapted for use with Opera's UserJS. A Free Greasemonkey guide (much of which will be applicable to Opera) can be found here. It appears that Opera and GreaseMonkey came up with this independently, though I'm sure the addition of a public interface for this for Opera users was inspired by GreaseMonkey's success.
Opera 8 also supports GreaseMonkey-style meta information comments like
@include example.com
@exclude private.example.com