Compete-tee-tion

Threadless
$1 minimum per item sold (more info).

Shirt.Woot
$1000 for the first day of sales and $2 per shirt sold after that point. Shirt designs can be submitted directly to Woot or entered in their weekly themed contest, the Derby.

Pampling
XPress winners earn 1 euro per shirt sold, with a minimum of 100 euros guaranteed. Classic winners earn 680 euros cash and 120 euros in t-shirts (or 720 euros cash) for the first 400 units and exclusive rights for 3 years.

Camiseteria
R$800 cash, R$500 in products

SplitReason
$350 and a shirt with your design

Purple Leaves
300 euros and 1 euro per shirt sold

Arctic Tees
$250 cash and a $50 Arctic Tees gift certificate

Monsieur Poulet
2 euros per shirt sold

WeLoveFine
Top prizes of $1000+ on a variety of pop culture themes, including a bi-weekly My Little Pony contest

These are sites sell shirts for a limited time, allowing the artist to keep the rights to their artwork.

Aplentee
$1 per shirt sold

Arteesel
$1 per shirt sold

blipshift
$1 per shirt sold (with a guaranteed minimum)

BlueBoxTees
$1.50 per shirt sold

Captain KYSO
$1 per shirt sold on day one, $2 per shirt sold after that

Global Thread Collective
$1 per shirt sold

GraphicLab Tees
$1 per shirt sold

Fresh Brewed Tee
$2 per shirt sold

Limiteed
£1 per shirt sold

MuzikShirt
$1 per shirt sold

NowhereBad
$1.50 per item sold

Once Upon a Tee
$1 per shirt sold

OtherTees
$1 per shirt sold

Qwertee
1 Euro per shirt sold

RIPT Apparel
$1 per shirt sold

ShirtBattle
$1 per shirt sold

ShirtPunch
10% royalty per item sold

Snappy Kid
15% royalty per shirt sold

TeeBusters
1.25 Euros per item sold

TeeFury
$1 per shirt sold

TeeNOW
9.04% of gross revenue

TeeTee.eu
1 Euro per shirt sold

teeVillain
$1 per shirt sold

TeeVolt
$1 per shirt sold

Tshirt Fight
$1 per shirt sold

Unamee
$1 per shirt sold

Weekly Shirts
$1.50 per shirt sold

The Yetee
$1.50 per shirt sold

Zebra Tees
$1 per shirt sold

Ze Tee
1 Euro per shirt sold

These are sites you can submit shirt designs or ideas to without having website visitors vote on them.

ThinkGeek
$250 for a t-shirt suggestion or $500 for complete, finished artwork through the Bounty program

Snorg Tees
$150 and a free shirt if they like your idea (see their FAQ for details)

Formamentees
Submit a design through the site's free submission form, and if curators like it they'll make you an offer.