ShirtMockup.com is a creation from the Go Media design firm and it makes mocking up shirts quick and easy. You can upload your design and scale it to the way you want it. They have two shirt options right now, but if you want more, you can purchase their templates or find other templates on the web. I personally like their templates and feel that they're very well done and look pretty professional with your design on it.
Here's a video of 1/3 owner & designer Jeff Finley discussing how to use this site.