This site provides an excellent beginners tutorial for Photoshop basics. The site also features some more in-depth tutorials, although some of them are out-of-date. Once you've mastered the basics of Photoshop, take a look at this site. They've got lots of good, intermediate tutorials that cover some cool effects and techniques.

Finally, check out PDSTuts. It's a great site with really high-quality tutorials. Most of the tutorials are pretty advanced, though, so I'd look at the other sites first.

You should never have to buy online tutorials for Photoshop. The free ones are just as good, and the paid ones are often a scam. If you want to spend some money, buy a Photoshop book instead. For me, it really helps when I have a paper reference in front of me. I'd recommend Adobe Photoshop CS3: Classroom in a Book. It was written by Adobe, so it's definitely high quality, and it covers both basic and advanced techniques .

Some web pages I like for photoshop tutorials are as follows:

http://hubpages.com/hub/Optimize-Your-Work-Flow---Understanding-the-Adobe-Interface

http://hubpages.com/hub/Making-Desktop-Wallpapers-in-Photoshop