Understanding relational and key-value store databases
While a title like Is the Relational Database Doomed? is obviously link-bait, this Read-Write Web article on the differences between relational databases and the current rapid rise of key-value pair data stores (particularly for distributed, rapidly scalable, multi-tenant web applications) is excellent.
Obviously, relational databases (like MySQL, Oracle, DB2 etc.) aren't going anywhere soon, but the use of key-value pair stores, as well as their recent resurgence with things like Amazon's SimpleDB, Google's Big Table, CouchDB and a host of others, and understanding where it might be a good idea to use them, makes this article a great primer if you've been wondering.
(note: this article covers a bunch of projects I hadn't heard of before including Project Voldemort, Drizzle and Mongo, so if you're interested in this area but didn't know about those ones, it is definitely worth the read)