Scriptish

Scriptish is a fork of Greasemonkey, offering:

Everything Greasemonkey does (which is actually useful)
Updating: User scripts can be updated in Scriptish using @updateURL
Many new metadata @keys
Many new additions to the GM_ API
Superior Performance: A faster, cleaner code base which takes advantage of all that Firefox 4+ has to offer
Superior Security: Scriptish provides security features you won't find elsewhere

If you like this add-on, then please consider contributing to development, there is quite a lot of work to do still!