I've completed a few on Coursera.
Gamification - Recommended, a really indepth look at the pros (and cons) of gamified systems. A thorough explanation that it isn't all about points, leaderboards and badges.
Content Strategy for Professionals - Not great, admittedly it was aimed at a different audience - I was more interested in the COPE model, and thinking of the bigger picture. It was touched upon, but very lightly - these were just lecture based talks, and it didn't appeal too much.
HCI - Good - The course itself was well done, and very worthwhile, however the coursework was ridiculous - you would need a lot more time to learn the tools before being able to use them. They wanted prototypes creating in tools I've never used before!
Social Psychology - Amazing - By far the best MOOC I have taken. This has already had a lasting impact on how I see things - fantastic tutor, fantastic in depth lectures, and thought provoking assignments. They are running it again later this year, and I highly recommend anybody with an interest in people to take this.
I've snagged all of David Travis' courses, on the general all encompassing subject of UX, his Ultimate Guide to Usability is the best I've come across.
User Expert Reviews - also by David Travis is again in the same vein, and very worthwhile. It looks at various design principles, alongside Nielsen's, and I found this very informative.
How to form habit-forming technology - by Nir Eyal, author of Hooked. Highly recommended, it goes into detail into the Hook model, and talks about how to engage users with a product so that it can form a 'good' habit. It looks at sites like Facebook, and Pinterest (in particular), and talks about what makes them work so well. The fact that the visitors have an 'itch' which is scratched by visiting these sites, and the fact that successful itches come from negative emotions - I am bored, I am curious, I feel silly... very worthwhile!
The Udacity Design for Everyday Things course was nice, but maybe I'm too much of a Coursera fan boy - I like their interface, and lesson structure much more. It seemed a bit light on the content too.