What are the best webs sites for designing shirts?


I don't know much about their design system (aside from knowing that they have one), but I have a shirt that was printed at www.customink.com and I can vouch for the quality of the printing and shirt material. If you have design skills you can use a free program like Pixlr to design the shirt.
Booster is a good one. Good quality tshirt. A local club tshirt was POS. Cracked at the first wash/dry with heavy itchy shirt made by Gilden I hate that company's product. Fades too fast as well while booster tshirt I brought two for a fundraiser and wear it frequently as PJs with no issues in quality or the lettering not being cracked its been 6 months now.