If you are on a very tight budget and don't want to spend a whole lot of money (or any at all), Picasa by Google is a simple but fairly effective editing tool. It doesn't have the options of Photoshop, Paintshop Pro or the GIMP, but it is fast and very easy to use. It has all that the amateur home photographer/happy snapper needs, so don't buy into the "I must have Photoshop" line, you can great results without blowing the bank or needing a three month course in how to use the program first! (Photoshop will not make you a better photographer either btw.)
Picasa also has a few extra little bits that make it easy to publish to Picasa's on-line web-album, resize for email, and even watermark, if you feel the need. Download it here: Picasa
Taken with a Nikon D90 and Nikon 35 f1.8DX, edited in Picasa.