Are there any specific features you're looking for?
Otherwise, for general photo-editing programs, here are a few:
1. Adobe Photoshop & Photoshop Elements
If you have money to spare, Photoshop provides a ton of expert features. Photoshop Elements is a lot cheaper but it also provides many of the features of Photoshop in a more user-friendly fashion.
Pixelmator is a bit cheaper than Photoshop Elements, but it also provides a lot of great tools. It's more for creating images from scratch and advanced touching-up, in my opinion.
If you're doing a ton of batch image processing, this is the program for you. Allows you to crop, rotate, resize, add watermarks, etc. on multiple images.
If you just want to create slideshows or do basic photo touch-up (e.g. adjust colors, add effects), iPhoto, which is included on Macs, should be enough.
Most (if not all) of the above software offer trials, so you can try it out before deciding.