There are many free UML tools available, some that work with Eclipse, some that don't.

1. Argo - mostly works, but poor export to Java; good for class diagrams; no sequence diagrams
2. Poseidon community edition - I haven't tried this, but I think it is single user only
3. EclipseUML (Omondo) - buggy: prevents Eclipse from loading sometimes; only works for single user
4. Umbrello - works well (access it under Linux at ~rth/public/umbrello)
5. Visual Paradigm community edition - good: only works for single user
6. Jude - very good; reverse engineering; all diagrams are complete (access it under Linux at ~rth/public/jude_community/jude)