This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
I have used poseidon, which has a community edition. Its great (for free) but can get a bit flaky as diagrams get big. I have also recently started using netbeans 5.5. Its still a preview edition, but its looking really good so far.
Im sure if you search the forums, you will find many posts on this already.