I think most people would say that Adobe Illustrator is the standard. I use Corel Painter for the drawing and Paintshop or Photoshop to separate the colors if I need to for printing. Painter is my favorite piece of software and I prefer Paintshop (if you can get version 7 of it, the later versions suck) to Photoshop. Paintshop is a lot more user friendly, I think. If you want to do vector art, you have to go with illustrator, though.
A forum community dedicated to T-shirt merchants, professionals and enthusiasts. Come join the discussion about graphics, commerce, dyes, prints, collections, guides, displays, styles, scales, equipment, accessories, reviews, classifieds, and more!