NHibernate
“Learn NHibernate” has been on my todo list for a while now, and I finally spent some time doing it today.
I must say I’m impressed. The folks who ported Hibernate from Java to .NET did a good job of making it feel like a first class .NET library, and not just a port. This introduction to NHibernate by Jason Gehtland is excellent (and here's part two), and with it I had my first classes mapped into database tables in just a few minutes.
I haven’t taken the item off my todo list because there’s still so much to learn about it, but I can see it’s going to be a huge time saver.