Here are some of my favorite photo editing tools:
Picnik.com - You can simply go into the website, upload a photo and edit it.
Picasa - This is a photo program by Google similar to Photobucket and Flickr. It has simple, built-in editing tools. I have it on my computer, as well as online.
Gimp - This is more advanced, but is like a free version of Photoshop. I used it to make my banner, and sometimes use it to edit photos. Usually Picasa and Picnik are enough, though.
With all these programs, you have to spend time to play around and get used to the effects. After a while, you'll be pretty fast!
Last night I took photos with flourescent indoor lighting and my photos looked yellowish as a result. I was able to edit them in Picasa and Piknic, and while they aren't perfect, they are the best I could do without natural sunlight. I'm not the best photographer, but the above tools have really helped me out.
Please share some of your favorite photo editing tools!