Basis of two most well-known proprietary Virtual Learning Environments.
Increasing number and popularity of FLOSS Virtual Learning Environments: Moodle, Saki, LAMS, Bodington.
Advantages: open standards, interoperability, customisable, flexibility, cooperative development. (http://www.elearning.ac.uk/features/oss#ref4)
Open Content (Creative Commons), e.g. MIT OpenCourseWare, National Digital Learning Repository.