Adobe OnLine has some good information and user groups also. Simple registration for the site and you can find tips, tutorials, actions, styles, link's to other good sites and also information from others in the groups.
Lot's of people here also know of link's and such as mr BoXXi posted

"his site is a Favorites URL keeper by the way".
Also, the Photoshop WOW series of books take you through some serious skill tutorials that can be applied in all areas of use of PS.
Such as taking line art, colorizing it, adding depth, and such, to turn out a photo quality graphic, cartoon style, painting and so on. Without using plugins, filters, actions and or styles, so you learn what to do and how to do it for the exact effect's your looking for.
anyway, just some thoughts...