This is a collection or UML programs I tried and my impressions of them. I was looking for the following criteria:

• code generation ability
• reverse engineering (turning code into UML diagrams)
• round-trip engineering, the ability to update diagrams from code and code from diagrams
• overall functionality, the ability to cut & paste portions of a diagram, print previews, XML exporting, etc..
• fast and responsive
• free, preferably open-source

I'd like to credit Objects by Design and their comprehensive listing of UML tools.

Poseidon Community Edition -- GOOD: Java code generation and limited reverse engineering. Unlimited license for non-commercial use. Cross-platform support (written in Java). BAD: Annoying "buy me!" pop-up everytime it's launched. No undo/redo or cut&paste functionality. Slow and a total resource hog.

MagicDraw Community Edition -- GOOD: Fast and responsive. Good functionality (cut&paste, undo, etc..). BAD: Annoying registration-by-email required in order to download & install. No code-generation or reverse engineering in community edition and diagram sizes are limited when saving to file. Complex licensing scheme.

Rational Rose XDE Modeler -- GOOD: It's considered to be the industry standard for UML development. BAD: IBM bought Rational. You have to register for an IBM Universal ID and then also fill out a survey about you and your company before you can download the trial version. The trial is 298MB and only works for 15 days. The installation is confusing and contains a bug that can potentially corrupt an NTFS drive.