Photoshop

Psdtuts+ has an interesting tutorial on lighting, specifically IES lighting.

In this tutorial, I'll show you how to create a realistic IES (photometric) lighting effect. We'll be using Photoshop CS4 as well as some free plugins and software. Let's get started!

http://psd.tutsplus.com/tutorials/tutorials-effects/how-to-create-a-realistic-ies-lighting-effect-in-photoshop/

Smashing Magazine has an excellent article on Photoshop's brush tool.
When laid by a learned hand, brush strokes can convey a sense of energy, tactility and humanness. These qualities speak to your audience's subconscious, whispering ideas that words alone can't convey. In the digital realm, a website with beautiful brushwork is a welcome break from the stark precision of most corporate websites.

http://www.smashingmagazine.com/2009/11/16/brushing-up-on-photoshops-brush-tool/