Dominiek ter Heide has put together a great tutorial that walks you through building a "Twitter agent" using Ruby, Rails and XMPP. For those who aren't on Twitter yet, it's a free micro-blogging cum "presence" information service that allows users to write 140 character messages that are supposed to represent their current state / feelings / location / etc. A Twitter agent, therefore, is essentially an automated Twitter user that can deliver information to other Twitter users.

The tutorial is quite in-depth with lots of code examples and helpful diagrams. Dominiek explains it all very directly, and even has the developed service running for real, so you know you're seeing something that actually works.