Another vote here for Nik. I bought their full suite of plugins for Aperture a while back (excluding the HDR one as I don't do HDR), and use them on all my photos. DFine is, in my opinion, the best noise reduction software out there, and being a huge fan of mono, I absolutely adore Silver Efex. The software doesn't do anything that can't be accomplished in Photoshop, but it does simplify the process by streamlining it, since you have all the tools in a single, consolidated UI. The u-point method is also quicker than painting masks in Photoshop, and generally works just as well. Viveza is great for quickly giving photos a bit more punch, while Colour Efex is pretty cool too although a lot of the filters are a tad gimmicky, and I don't use it as much as the others. Sharpener Pro is decent although I sometimes still use Photoshop's Unsharp Mask instead, as it has a particular look that I'm rather fond of.
I'd say DFine is an essential purchase if you're regularly shooting in situations that require high ISO, and Silver Efex is the best mono processing app out there for digital shooters.