A few weeks after Theory11 released the Wire (Now known as the Marketplace) Penguin released their Penguin Partner program. Anyone can create a profile at Penguin but the submission process is not the same as Theory11.
Theory11 has made it very easy to create an a account but they have a series of checks to make sure that what you publish is original. Their standard of originality is very high, many products that are released commercially would fail to be published on Theory11's site.
Penguin is a little difficult to get into initially. You need to correspond with Shaun Dunn. He's going to look at you as an artist and treat you almost like a dealer. When he establishes that you have something to offer they are very relaxed as to what they allow you to submit. It is all mixed in with their various products and you have to compete with whatever the big trick of the month is.
Penguin also offers in store credit and Paypal direct payments. They also allow you to publish books.