[fractallyte]

OK, now that someone's mentioned Expression, I'll note that I use it extensively - the original version, that is: http://en.wikipedia.org/wiki/Creature_House_Expression

Aside: Creature House Expression was a wonderful - yet industrial-strength - 'painterly' vector drawing package. Superb UI (which many users complained wasn't 'Adobe' enough), and a fun, quirky website. The combination of science and art made it one of the highlights of successful software development. You can guess what happened when Microsoft acquired the company... ;-)

Anyway, MS were kind enough to make the Windows and Mac Classic/OSX versions available for free - follow the wikipedia links.

Just remember, this is software that was a rival to Adobe Illustrator, and contained technology that some thought would threaten Flash. Steep learning curve (but excellent PDF manual included), and capable of far more than UI mockups. The old Topica mailing list is a mine of useful information, and Yahoo has a current (but less trafficked) list.