On-line Learning Tools?

As a follow-up question to last year's query about distance learning tools, denny_d asks: "I've been looking around for awhile for an OS e-learning tool. Been using Viewlets for awhile and recently tested Claroline which looks very promising. Are there any other OS apps I should know about for creating learning/training environments on the web?" Have there been any new technologies or projects that have made a big splash in the Distance Learning scene in the last year? What can teachers looking to implement such courses look forward to, and what issues might they have to watch out for?