MozRepl lets you program Firefox from the inside
wiki.github.com
wiki.github.com
It's quite neat, but very easy to get confused: if you switch to a different tab to read documentation for example, then make an edit and save, it will try and load into the new tab (usually with strange results!)
http://ted.mielczarek.org/code/mozilla/extensiondev/
It had other features, but the REPL was the most important to me. This looks much nicer though.
In fact I would say it's a kind of a pattern: implementing REPLs for systems that weren't made with REPLs in mind.
(That doesn't say anything about the coolness of this hack though, one way or the other :)
On one hand, it's built on top of Firebug; this adds a lot of power (chrome inspection, javascript debugging/breakpoints in the chrome).
On the other, it's alpha, and not the focus of development (as far as I can tell)--it hasn't had a new release in some time.
Edit: Just realised that this is pure javascript, making it a lot easier to deploy than jssh, and also a lot more extensible. That is awesome.