Pluralsight, Udemy, Lynda, Coursera, Code School, Stack Skills
I've also used some small independent ones as well (Bitfountain is just one example).
The thing I would be really curious to see (but unfortunately don't think we will see any time soon) is how the profit numbers stack up on these different pay models.
For the subscription based services I will pay a monthly fee for no more than one service at a time and I will make heavy use of it during that time period. This is what I did with Pluralsight, Code School, and Lynda. Got a subscription and used it for awhile then canceled when I was done with the material I was interested in or I didn't have enough time to spend for it to be worth the money.
I also cap the value at about $50/month max. Not that Udacity has bad content, but there is great content on the competitors too. I can sign up for Pluralsight + Lynda + Code School every month and still have money left over to buy some courses outright on Udemy or Stack Skills.
In total I've spent a lot more money on buying courses outright from sources like Udemy. Originally I was looking there for some specific technical classes, but they have so much variety I have spent more money on things like art, photography, and design classes.
Wow... I went off on a bit of a tangent there. Anyway; really would be interested in seeing how much money is being made by these different services as well as seeing percentages/dollars of how much of that passes through to individual content creators etc...