Canvas is.

http://www.instructure.com/

We use it at my school and it's great. I also use their api as a developer at EmeraldExam.com and find it to be terrific. They're an open-source rails app: https://github.com/instructure/canvas-lms

EDIT: should also mention lore (lore.com), which was founded by some friends of mine and fairly recently purchased by a John Katzman company.