Valve has put up a character design guide for their upcoming game Dota 2.
The guide is in the form of a free 18-page PDF that covers concepts like silhouette, colours, values, character poses, and other stuff.
It's not an in depth tutorial guide but more of quick tips on improving your character design. There are also some examples of game characters from Valve themselves, of course.
The goal of the guide is to enable artists to use those tips to create in-game assets which they can then sell and earn a few bucks. Nice.
You can download the character design and texture guide at www.dota2.com.