Here's two I use a lot:
http://www.digitalmastery.com/tips/
Ben WIllmore writes for
Photoshop User magazine and teaches Photoshop professionally. There are a lot of good tips here.
http://www.worth1000.com/tutorial.asp
Most of the stuff here is for photo-manipulation, but even if turning Catherine Zeta-Jones into a cyborg isn't your thing, you can learn a lot about HOW Photoshop works by following along with the tutorials.