jQueried is a Safari 5 extension in the same vein as the jQuerify bookmarklet. jQueried adds jQuery to the current webpage, a task which can be very useful for debugging.
After finishing my first extension, simple as it is, I am impressed with the simplicity of Safari's extension API. Coupled with Firefox's Jetpack and Chrome's extension system, it looks like the future of browser extensions is all JS + CSS + HTML.
Source is on github: https://github.com/kyleconroy/jqueried
Download the extension: http://dl.dropbox.com/u/40773/jqueried.safariextz