In the past, at companies that didn't yet have a robust automated testing suite, I've used Test Run to manage manual test scripts for multiple users. It was a pretty handy tool, but the site now seems to be defunct - upgrading past the free trial is broken and nobody seems to be answering support questions. Before I go back to copy and paste with a spreadsheet, I thought I'd check to see if anybody here knows of a similar web-based tool that is actually maintained.
It doesn't necessarily have to have every feature that Test Run did, but the ability to write test scripts, assign them to different users, and create one-off test plans from existing tests are pretty key.