[uml] What's the best UML diagramming tool?

Rational and Together/J are best-of-breed products, but expensive.

In my experience, I've enjoyed Eclipse Omondo and Sparx Enterprise Architect. Omondo integrates nicely with Eclipse for code generation, and has a very intuitive feel. However, it is strongly tied to Java. Sparx is a good tool for the price point, but lacks the full range of UML 2.0 diagrams.

Do NOT bother with Poseidon. It is buggy, bloated, and unusuable for all intents and purposes.