Moodle is the premiere—AFAIK, the only interesting—open source "Learning Management System" (is that the current buzzword?). I've been running a Moodle instance at Portland State for many years now: counting the current one, I've taught 14 courses with it. It has, overall worked OK.
My alternative to Moodle would have been to use the school's infrastructure. PSU used WebCT/Blackboard when I started, and now uses Desire2Learn. WebCT was terrible, and nothing I've heard about D2L has given me any real motivation to give it a try.
However, Moodle is a giant, ugly mass of ill-thought-out PHP code, and occasionally it gives me hives. I was up until 2PM last night, 1.5 hours past my bedtime, trying to figure out how to get Moodle to both allow guests to view my new course and allow students to self-enroll. I've tried this before, but my latest strategy was to upgrade to a recent Moodle (2.5) and hope it fixed things. Nope. Now I'm going to put up with the same workaround I've used in my last few courses, and eventually probably start reading PHP in the hopes I can find and fix the bug. Yay.
Oh well, Moodle's free and it mostly works. What more can I ask? But I'm sure feeling ambivalent. (B)