White board it, modeling should be a group activity. Everyone should contribute their ideas so we can converge towards one mental model. If I'm working with 1 or 2 developer, we can cluster around a table and use pen and paper. I only use professional tools if I'm going to be giving presentation or wish to store it for long term.

In Linux there is modelio, and there is also Visual Paradigm which is available on most platforms and has a free community edition.