This week's book giveaway is in the Agile and Other Processes forum. We're giving away four copies of Real-World Software Development: A Project-Driven Guide to Fundamentals in Java and have Dr. Raoul-Gabriel Urma & Richard Warburton on-line! See this thread for details.
There are many tools that do this; some are better than others, cheaper than others, etc. I happen to like EssModel for building class diagrams because it's free, very fast, and generates good quality models.