I'm stuck with a big chunk of 'Free Space', how do I restore my Macintosh HD to a single partition?

I've recently deleted a Windows partition from my Mac, and I've somehow been left with a giant chunk of 'Free Space' as a partition on my harddrive. I can't delete it or scale it down, and I'm left with no options.

Does anybody have any ideas why this might be happening? I'd really like to get that 320GB back...