Photoshop, Lightroom, Photomechanic, and Breeze Browser are all programs that I've used for PP. My favorites are Photoshop CS2 (or the new CS3 Beta) and Lightroom. If you need to do quick edits Lightroom is great. If you need to do some more in depth processing I prefer CS2.
Post processing is stuff you do after you have the digital file (scanned film or capture with a digital camera). It can be as simple or complex as you choose to make it...