I've just put the finishing touches on the 1.0 releases of the Signup Scheduler and Signup Status modules, which I've written about before. While they're both in just their initial release, I feel pretty good about their stability and utility. We've been using both modules in production on the IDEAL site, where users can register for professional development courses. I've also been getting lots of great feedback and bug reports from community members in the issue queues. Special thanks to Mark Dowsett for testing and issue reporting and Derek Wright for working with me on some helpful changes to the Signup module.
The project pages have plenty of details, but most notably the Signup Status module contains lots of goodies that weren't mentioned in my original post, such as plenty of Views integration, signup certificates, signup status change emailing, and more. Check it out if you're interested!