AutoWifi is a pygtk app which sits in the notification area waiting for a known SSID (of free hotspots) and logs you in automatically, so you don't have to fill webforms yourself. The first time you access a free wifi, AutoWifi will prompt you in a gtk dialog for your account credentials of course.

AutoWifi works for FON hotspots and FreeWifi hotspots and it's a brand new project so if you can help its developer (he especially needs help testing AutoWifi outside Europe (for fon spots)), see his post @ ubuntuforums.

Installing AutoWifi

For Ubuntu, there is a PPA available (only for Ubuntu Karmic):

sudo add-apt-repository ppa:manatlan/ppa
sudo apt-get update && sudo apt-get install autowifi

More information on AutoWifi.