Scott Hanselman is using Watir, an open source Ruby library, to do his Web App testing. He says it's "the bomb." It wires together with NUnit by spawning a new process on Ruby's shell to run asserts from Wasir. Looks pretty neat.
Hanselman also points out how to use Firefox's Web Developer extension to help out. Another +1 reason why Web Developer's so cool.
I've got some upcoming work which will need some web testing. I'll be sure to look at this!