Converts urls to is.gd keeping them as short as possible.
Only issue with this is it's changes urls as you type, so if you paste a url then continue to type after it, it embeds the url and the text you have just typed all into the same url.
So never paste the url in until you've typed everything you need to, even if it's at the start of the message.
I have tried two other scripts that are supposed to do url shortening in twitter and neither of them worked. This one simply does work. By the way I am using the latest version of Firefox (version 3.0.10) on the Windows platform - maybe this is why the other versions didn't work but who knows?