My day job is as a software developer, so I'm often grabbing screenshots of new UIs we've developed. Resolution isn't important for my usage, but I'd imagine it'd be comparable to Snapz.
I don't use Mac, but for PC I've personally used both Jing and PicPick. Both are free. I typically use Jing, just because it's slightly easier and faster to use in most cases, but PicPick allows for more editing of the capture.
I have some coworkers that use SnagIt, which is a paid software made by the same people that make Jing. It's quite a bit more powerful, from what I've heard.