I'd also recommend Acorn[4]. It's a phenomenal app, has recently received a major update (early bird pricing until end of May) and is, in my opinion, much more "Mac-like" and pleasant to use compared to Pixelmator.

It's my go-to app for fast tasks while still being powerful enough to do more serious things. Not to mention it has rock solid scripting support too.

[4] http://flyingmeat.com/acorn/