If you are programming in Java 5.0, I strongly encourage you to take a look at the Google Collections Library. I'm really impressed at the amount of work that went in to crafting this library.
This project would not have been the least bit possible without Kevin Bourrillion's hard work. Creating this massive library was his idea, and he managed to get several dozen engineers to contribute features, code reviews and tests. Kevin is also partly responsible for Google Guice.
I'm not going to bother going into any details about specific library features, I think Jesse Wilson is doing a fine enough job at that with his gentle introduction. I'm already drafting some posts to go on his blog.