You could use the built-in Ruby SDBM persistent hashing solution. It's extremely fast and will run under Rails or any Ruby application on any web server. The best part is it's included in Ruby - no plugins or gems!
I covered it on one of my earlier Rubyology screencasts (http:// rubyology.com/mp3s/rubyology25.mov).
Cheers, Chris Matthieu http://www.rubyology.com