This website has a TON of information about how to do things in Photoshop. I only know the basics of Photoshop, but I am a very fast learner – so glancing through a few of these tutorials was really interesting to me.
This website has 50 different tutorials. The ones I found most interesting were the ones dealing with Lighting Effects (#7), Silhouettes & Gradients (#9), Pencil Effects (#10), Smoke Effects (#26), Worn Vintage Posters (#27), and Custom Patterns (#36).
Each of these tutorials goes very in-depth and step-by-step, with a lot of photos to help the reader understand. In each tutorial, multiple tools in Photoshop are utilized, so you might be learning multiple tools rather than just one effect.