I just wrote a little tutorial about how to implement a Twitter filter web app using Sinatra, Redis, and TweetStream. It discusses both the architecture and rationale for my technology choices, as well as presenting the actual code. You can see my example app live at twatcher.com. Hope this is useful.

I posted a comparison between two interesting Ruby on Rails hosting providers: Heroku and Morph. Both of these significantly lower the barrier for getting a new Rails based website off the ground.