I use the Gimp (Gnu Image Manipulation Program - http://www.gimp.org/). It can do most of what Photoshop can do for common tasks and more. Plenty of documentation available as well as tutorials on the web or YouTube. And it costs nothing!

Pros will probably go for Photoshop or Lightroom as they are more likely to use those few missing features and can afford to pay for it.

I'm wondering what sort of editing you're trying to do. Are you looking at advanced stuff suited for Photoshop or more basic tweaks and workflow like Lightroom?

I'm somewhat skilled with Photoshop, but I rarely even use it anymore after getting Lightroom. You can get a student edition of Lightroom for $100. I love it. Even if you're looking for more advanced editing capabilities, download the 30 day trial of Lightroom from Adobe and see what you think. If you're not already using it, you'll want it. It's fantastic.

Thumbs up for Lightroom, I converted a couple of years ago. It has great features for newbies and pros alike.

In the newbie category, I've often used picasa (picasa.google.com) in the past. It has nice basic editing features, and makes it very easy to share photos.

That said, I've been interested in trying piknik, which is online. It must be pretty good, as they were acquired by the Google monster just a few days ago.