For freeform stuff (usually website mockups), I'll usually just grab a piece of paper or use Inkscape.
Inkscape is probably a decent tool for thinking (at least if you're already familiar with it), since you can zoom in and out on an infinite canvas, draw and type things, etc.
Inkscape is probably a decent tool for thinking (at least if you're already familiar with it), since you can zoom in and out on an infinite canvas, draw and type things, etc.