Balsamiq Mockups is very good.

At my new gig, I spend a good bit of time mocking up wireframes for a new web app. I tried Omnigraffle, Powerpoint, Keynote, and finally settled on Balsamiq Mockups. It really is a great program. It's simple and hyper-focused on one task - software mockups.

The user interface is trivial (you drag and drop screen elements onto the page and move them around). The learning curve is zero. But the best part is that the mockups are styled to look like hand drawings. At first I thought it was an unnecessary gimmick, but as I began to use it, I really appreciate the feature.

Seeing the mockups with their rough, hand-drawn look reinforces that they are mockups, not cast-in-stone specs. That is a very powerful communication tool when dealing with clients or bosses who want progress updates. You don't have to preface everything with "this is a mockup, not real software", or "this is a crude alpha" (how crude?). It's really great - people instinctively understand the stage of development just by looking at the mockups.

Balsamiq is a little buggy, but not in a way that stops you from working. And I wonder if that has more to do with Adobe Air than Balsamiq. In any case, I highly recommend it.

Note: The makers of Balsamiq give a way free copies to bloggers who write honest reviews of the product. I did not take advantage of that program - I paid for my copy and liked it enough to tell the world about it. I also have a selfish interest in that I want to be able to share Balsamiq files with others, which won't be possible until everyone has Balsamiq...