This week's book giveaway is in the OCAJP forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide 1Z0-808 and have Jeanne Boyarsky & Scott Selikoff on-line! See this thread for details.
You can try Astah(previously known as jude).
Make sure that there is no watermark generated by the tool(some of the tools which are commercial-there community version does generate the watermark).
Visual paradigm is very good - one can create animation out of the sequence diagram also. Far better then free or open source tools.