Nerdy Shirts offer $200 cash, $100 store credit and Fame for t-shirt designs that are accepted. The incentive is nowhere near as high as at Threadless.com but I imagine simpler designs are accepted and Nerdy Shirts seems to focus on pop culture rather than abstract design so if that is your field that is where you should go. Below are the details from their web site.
1 ) Submit Your Design
Have a great idea for a T-shirt? Create your design and submit it.
2 ) Design Approval
We will internally check each design to make sure it meets the design requirements. Upon our seal of approval, we will release it to public voting.
3 ) Public Voting
Upon approval, your design will be released to be voted on by the NerdyShirts community for one week. During this process, you can promote your design and check on its status.
4 ) Win!
We will personally review all top scoring designs and choose the winners based on score, positive comments and quality. Winners will receive $200 cash, $100 in NerdyShirts store credit and of course your brand new designed t-shirt.