There seems to be increasing interest in UML these days, at least in the circles that I swim in. But not everyone who's interested can afford the high-end price tags of Rational XDE or Visual Studio .NET Enterprise Edition. But there are other, less expensive alternatives out there. One of these is Enterprise Architect, which packs a full UML IDE into an affordable package.
The IDE is reminiscent of Visual Studio .NET, though it's an independent shell. Inside of it, you get a treeview Project Explorer that shows you how everything fits together—you can drill clear down to attributes and operations on classes. All of the standard UML diagrams are here (use cases, class view, activity diagrams, and so on), with a drag-and-drop environment for constructing the bits and pieces. There's quite a bit of help on using the IDE, though if you have experience with any modern IDE you probably won't need it.
Depending on which edition of EA you purchase, forward and reverse engineering are also supported. You can move from your UML to Java, C#, C++< VB.NET, Delphi, or VB6. I spent a while moving from a model back and forth to C# source, and the operation was simple and foolproof in both directions. There's also XML-based import and export, including support for the XMI specification that can be used to exchange information with other UML tools.
Other interesting features include the ability to support multiple developers, either in the same project or in replicas that can be later synchronized, and the ability to store models (depending on the version of the program that you're using) in Jet, SQL Server, or MySQL repositories. The help file includes upsizing instructions to help you migrate if you start with Jet and later outgrow it.
You can download a trial copy of EA from the company's Web site, which also includes a bunch of UML tutorials to help you get started.