from Lifehacker by Adam Pash
Windows/Mac/Linux (Firefox): Script repetitive web applications—like filling out forms and paying your bills—with the CoScripter Firefox extension. CoScripter is very similar to the previously mentioned iMacros extension but offers users a much friendlier interface for creating new macro scripts, meaning you shouldn't need any programming experience to create your own scripts (be sure to check out the video demo on the site for a good introduction). CoScripter is free to download (though it requires an unfortunately convoluted registration with IBM), works wherever Firefox does.
CoScripter