Moodle

I'm a bit of a newbie when it comes to Moodle, but what I've seen so far, I like. I was never particuarly keen on it as Moodle forums always seem to be full of participants talking technobabble about xml, javascript and other such mysteries. Having taken the time to actually look at the manual, create my own Moodle site and tinker a bit, I would say that it is not for the faint-hearted or time-pressed teacher. It has amazing functionality - after all, it is a free and open-source tool that literally thousands of great minds are working to build - but it can be somewhat overwhelming, and I would say that to get the most out of it, you need at least basic coding knowledge, even if it's only a bit of html. As the average teacher has little time to become proficient in such things, it is possibly best for those who are serious about building complete online courses, rather than supplementing 'face time' with an online presence. 
If you've got the time though, there's plenty to keep you busy and entertained. Warning: It's very addictive, but also hugely time-consuming initially. However, once you're versed in the basics, the possibilities are endless, and when you do reach the end, you can just write more code to make it do what you want. A long-term investment of time and energy that will be worthwhile for some, but definitely not all.

[For alternatives, follow the Learning Platforms label to other featured tools]