Because I couldn’t find any current lists out there. Right now I’m just posting ones that I like that are fresh in my mind. I’ll update with more in the future.

Design By Humans
Prize: $1,000.00 USD
Winners: Shirt of the Day Winners awarded Mon-Th by staff pick.
More Info: They print a lot of artsy all-over-the-shirt designs.

Goodjoe.com
Prize: $500 + 1.6% commission on sales and store credit
Winners: 2 winners chosen weekly by the staff
More Info: The “ongoing contest” is on its way out. Don’t submit to that one. You can join their Designers Community on Facebook for updates and to interact with the staff and other designers.

LaFraise.com
Prize: €1000 (about $1500) (artist regain non-exclusive the rights to their design after a year)
Winners: 2 Winners chosen weekly by the staff
More Info: They’re french!

Qwertee.com
Prize: €1 per shirt sold (artist retains the rights to their design)
Winners: New designs go up for sale every couple of days.
More Info: They mostly print pop culture designs.

Shirt.Woot.com
Prize: $1K + $2 per shirt sold after the first day of sales.
Winners: 3 winners chosen weekly by votes in their themed “derby” contests.
More Info: They also accept daily submissions that are submitted directly to them.

Springleap.com
Prize: About $500 for their regular contests, up to $1500 for sponsored contests
Winners: Every two weeks chosen by votes.
More Info: They’re South African!

TeeFury.com
Prize: $1 per shirt sold (artist retains the rights to their design)
Winners: Daily staff picks.
More Info: They mostly print pop culture designs.

Threadless.com
Prize: $2000 + $500 store credit
Winners: Multiple winners chosen weekly from a huge pool of designs.
More Info: It’s more difficult to get printed here than at most of the other sites, but you’ll feel good when you do.

TShirtContest.com
Prize: $1 per shirt sold (artist retains the rights to their design)
Winners: 1 Winner chosen weekly by votes
More Info: This contest is very new, so it’s an opportunity to get in at the beginning. You can join their T-Shirt Lounge on Facebook for updates and to interact with the staff and other designers.