My favorite online workshop is Critique Circle. I've gotten to know people there, well enough to feel bad when I'm absent for several months, well enough to know their work and look forward to it. I'm shy and not good at getting to know people through chit chat, but reading someone's book give you a glimpse of their hopes and dreams - like reading their blog.
All you have to do to earn credits is to write a review of at least 300 words for a story that is at the front of the queue. When you've done several of those, you have enough credits to upload your own story. It will probably get reviewed, since your first three stories go in the newbie queue, and the workshop is structured to encourage people to review the stories at the front of the queues, which pays more credits.
It might seem it doesn't matter much if your review is good or bad, as long as it's over 300 words. There's an informal system inside the formal one though. Anyone can write useless reviews - but if you write a very helpful one the recipient is likely to review you in turn. They might even go back and review your older chapters, though they get fewer upload credits for doing so than they would for reviewing the most recent material.
If you subscribe you can create your own queue with your own rules. So far I haven't done this yet - I'm saving my money to publicize my book after it gets published. They've placed limits on the personalized queues, so as not to make the free users feel too frustrated. The large user base is one of the major assets of this workshop, because it makes it much easier to find someone who is helpful to you, and considers your critiques helpful to them as well.
What's your favorite online workshop? What do you like about it?