There is also DecoNetwork that is similar to InkSoft - in price and the fact that your websites can be created on their system.
I use Deco, and I really like it. The biggest complaint I have heard about them is that their customer service is below par, but in my experience anytime you open a support ticket it gets answered pretty quickly, and they're willing to really go pretty far to fix the issue, which is nice. They also have a small forum, and the few of us who participate there try to help out as much as possible.
I haven't tried any of the others, but I have enjoyed my experience with Deco so far. The big downside is that they don't offer a free trial or anything, so you really have to commit to a month before you will get a feel for if you like it.
I do think that InkSoft and ShirtTools are the other two big ones...OpenTShirts seems like a really neat idea, too, if you are into that. If you have enough skill, I imagine you can do some pretty awesome things with that software.