Here are the ones I looked at when creating my own custom tees (in alpha order):
Note: I have to say I like this site, if the cost wasn't so prohibitive I may have used them.
Pros: Simple, easy, fast
Cons: Can't manage designs, older designer app
Note: I have used these guys for years and they are great at what they do. They aren't the cheapest, but their prices are fair, they have great customer service, 2-week guaranteed turnaround and free shipping. I wish I could manage my designs, however. They also allowed me to print everything, as long as I confirmed the shirts were to be used as gifts.
Pros: Good for people wanting to sell their designs/products
Cons: Confusing web site, designer app not as good as others
Note: You need to agree that your designs are not copyrighted (which is fine, but one doesn't always know). Also, the site focuses on the design, so you have to apply the design to a product (like a t-shirt or mug)
Pros: Simple, easy, manageable designs, costs inline with CustomInk
Cons: None, really
Note: If I was allowed to print both of them, I probably would have. They were very nice about the copyright issue and did the necessary research for me. I did get one shirt done by them that they determined had no copyright issue.