Seriously. A whiteboard, post-it notes, and lots of pencils and paper.
I'm a firm believer in UML-as-sketch (vs UML-as-blueprint), and for sketching, nothing I've seen beats the old school.
EDIT: fixed punctuation
- Rearrange easily (no copying)
- Support text and diagrams
- Are concurrently editable (as a group)
- Convert virtually useless walls into huge work surfaces
The idea that really hit it home for me is The KJ Method (aka Affinity Diagramming aka Snow Carding). Here's one description: http://www.mindtools.com/pages/article/newTMC_86.htm
There's a lot of way of organizing ideas and meetings using them that are really non-obvious. This (short, sweet) book covers some: http://www.amazon.com/dp/1555611427 Coders will get the ideas after the first chapter, but others should find the rest of the book educational as well (plus give some insight into basic data structures).