Sphinx and full text search engines
Great intro and springboard article from the Register if you don't know much about Sphinx on the smarter, scalable way to do full text search these days. All the cool kids are using it (and with good reason).
Sphinx and Rails goes together pretty much like chocolate and peanut butter these days and we're using it on at least two production projects at AI, as well as it being a favourite of mine for personal use.
The article is particularly good in showing how Sphinx evolved in response to search engine issues from using MySQL full text search and Apache solr and lucene in other places. Immensely useful to understanding the often murky world of search and information retrieval dark arts and why you want to use sphinx instead of just jamming in whatever seems like a safe bet for what is often one of business' biggest beef with database systems (getting stuff back out of them). Particularly since scalability and relevancy of results are seemingly becoming increasing issues with both of the more traditional solutions to these problems.
Worth your time to read if you're not up on Sphinx. I've been very surprised at how very few people seem to have heard of Sphinx outside of the Rails community.