On non-XP machines, a user has to provide their credentials via a web-based form before access to the network is granted. I have a MacBook Pro, and I've gotten really sick of having to re-login to the campus network (both Ethernet and wifi) via this webform every time my computer goes to sleep or I change locations. So, I've managed to write a bash script that uses curl to log me in through the web forms.
Thanks to some amazing work by macrumors forum member Wombert, I've also found a way to have it activate automatically, any time the campus wifi network SSID is detected. This automation avoids heavy-handed cron jobs by leveraging two OS X features, configd and Kicker.xml. (There's a previous hint here that relies on AppleScript and iCal scheduling.)
Now the script and Kicker.xml transparently log me onto our campus network, and hence the internet, any time my computer senses a preset group of SSIDs. The curl commands are specific to Cisco Clean Access, but they could easily be adapted to other environments where users must login through web forms, making this script potentially broadly useful.
One downside to the script right now is that it stores the username and password in plaintext. Perhaps someone could make this part more secure with more sophisticated code. Instructions are included in the comments of the script. To use the script, just copy and paste into a .sh file, and make it executable. You can also see it all marked up at pastebin.com.
[robg adds: I have not tested this one...]

