If you're a Lightroom user, you'll want to visit this terrifically informative and useful blog by photographer Matt Kloskowski. It's full of great lessons and free presets for download.
Adobe Photoshop Lightroom Killer Tips
If you're a new user, of just want a quick hit of ten solid tips, you may be interested in this NY Times article.
Ten Photo-Editing Tips From a Pro - NYTimes.com
