I was recently approached by software developers who are looking for free UML tools. Assuming that you are looking for a tool that supports most of the UML 13 diagrams and assuming that you are looking for a tool that apart of the diagram it creates the model and allows you to generate code, I recommend on checking the following options:
Astah (http://astah.net/)
You can use the free community version, enjoy a free limited version for your iPad and enjoy a huge range of plugins developed by a growing community world wide. The tool itself sticks to OMG UML specification and is highly comfortable for use.
Visual Paradigm (http://www.visual-paradigm.com)
You can use the free community version, either as a stand alone application or install it as a plugin for your Eclipse IDE or your Visual Studio. The tool itself sticks to OMG UML specification and is highly comfortable.
Star UML (http://staruml.sourceforge.net/)
The Star UML tool was developed as a free open source project. Although the last version was released in 2005 its implementation for the OMG UML specification is still relatively good (comparing with many other tools, as Microsoft Visio, that nearly ignore OMG).
