Inprnt - Art Prints
RedBubble - Prints, t-shirts, and a wiiiiide range of other products. I update Redbubble the most.
Society6 - Similar range of products to Redbubble.
DesignByHumans - T-shirts, phone cases, stickers, and a small range of other products.
TeePublic - Primarily shirts, with limited other products. Frequent sales with tees as low as $14!