I am not going to bore you with listing them all (because you really should go take a look), but I have done a handful of these tutorials and will share the results of two that I have done below.
Before you say things like "I don't want to give my client a tattoo", "surrealism is not part of my business model", or "Scott, you really are the smartest guy I know" (well okay, no one will say that), but before you say the first two realize that doing these kind of tutorials will have you using layers, brushes, and techniques in new (or different) ways that just might make you a better user of Photoshop.
And what starts out as a good learning tool could very well turn into a fun little project for a friend or family member.
Here are my two examples, the first is the "crack and peel skin". For this one you will need a background images, I used rust and dried mud, both of which I got (for free) at CG Textures.
The second is the "tribal tattoo" and I don't remember where I got the tattoo, sorry.
To get a better look simply click on any of the images to view larger and in a new window.