There's also Moom [1], but the reliance on arrow keys for most of the key-based functionality is annoying. I've mailed the author to ask if this could be configurable (so that I could use hjkl instead), no reply as yet though.

[1] http://itunes.apple.com/us/app/moom/id419330170?mt=12

Edit: Developer replied, turns out this functionality is already there - problem solved (for me)!