I ran into Balsamiq Mockups last Tuesday at about 11PM. By 2AM I was placing an order, with bleary eyes. Balsamiq is just that much fun to play with.
Balsamiq is just $79, runs on Mac, Windows, Linux, and in a web browser, and is a straight-up joy to use. It’s quicker than drawing by hand, with a much better end result. It auto-aligns objects, offers easy customizations in most of the ways you would expect (and a few you wouldn’t), and supports re-layering and multiple levels of grouping.
The only obvious shortcomings are minor: there is no way to add to the library of controls — they add more upon request, and you can import an image if you like; and it seems as if it may have a slow memory leak — after an hour or so, it seems like saving, exiting, and re-opening is a good idea because it’s slowed down a bit. Peldi, the creator of Balsamiq says the latest version should improve this (see his comment below); I’ve just downloaded it. Both of those are minor compared to the sheer exhilaration of how perfectly focused this product is.
I should also mention that I took Balsamiq in to work with me the next day. Our Product staff fell in love with it immediately. They pretty much demanded that their boss buy it for them, and just like that it’s the tool of choice at work for doing product design. I’m not kidding, it’s just that good.