Windows & Mac OS X: Photoshop
Photoshop is a great program, but it’s overkill for most people, who just need to crop, rotate, and perform simple edits to images before they upload them. And it’s also very expensive—around $20 to $50 per month!
Windows & Mac OS X: Photoshop Elements
Photoshop Elements is a simplified version of Photoshop. Some people like it and some don’t. It’s a lot cheaper than Photoshop ($70 at Amazon), and it’s a lot easier for “normal” people to use.
Windows: IrfanView
IrfanView is free, and you can download it at http://www.irfanview.com. It has a fantastic list of features and will do just about anything most people need!
Mac OS X: iPhoto
iPhoto comes free with every Mac, and it will do the absolute basics (and then some) very well, but if you need something more sophisticated, you’re going to need to look at the other programs on this page.
Mac OS X: Acorn
Acorn is a step up from iPhoto and a step down from Photoshop. However, it’s a lot cheaper than Photoshop—only $50—and it’s a lot easier to use. You can download Acorn using the Mac App store or on the Web at http://flyingmeat.com/acorn/.
Mac OS X: Pixelmator
Pixelmator plays in the same space as Acorn. It’s an excellent program, and costs just $30, but you can only buy it through the Mac App Store. You can read more about Pixelmator at http://www.pixelmator.com.