There are many great software options available. Photoshop CS is the cadillac of photo editing. It's also very pricey and complicated. Some other options include:

Free

Picasa...from Google. Provides basic editing and excellent organization
Ifranview...More advanced options
Photofiltre...another more advanced program

Online Options

Picnik
Splashup
FotoFlexer


Paid

Photoshop Elements... a less complicated sibling of Photoshop
Tutorial