Khan Academy — Education non-profit that develops practice exercises, instructional videos, and a personalized learning dashboard that empower learners to study at their own pace in and outside of the classroom.
Coursera — Massive Open Online Course (MOOC) offering many, many free courses.
EdX — MOOC that offers 200+ online classes from almost 50 partner universities.
Lynda — Professional online courses, classes, training, tutorials.
Memrise — Uses game-based and visual content to help users boost skills in vocabulary, languages, history, and science. Includes 300,000+ courses.
MIT OpenCourseware — The ability to audit 2,200+ courses from MIT.
NovoEd — Social learning and professional development.
Skillshare — Online classes both paid and free.
Tuts+ — Free how-to tutorials and online courses.
Udacity — Free online courses.
Udemy — Many online course topics.
Cell-Ed — Online learning courses optimized for mobile formats (even feature phones).
Good-Tutorials — Offers technology-focused tutorials curated from third-party sites on CSS, HTML, photography, Photoshop, and more.
Learni.st — Learn from expertly curated web, print and video content.
PSDTUTS — Free tutorials in design and illustration, 3D, motion graphics, photography, and audio.
TechChange — Washington, DC-based education tech startup that provides online professional development in technology and social change.
MicroMasters — Course in 'Data, Economics, and Development Policy' from MIT's Banerjee and Duflo.
Open Educational Geography Resources — Wide range of free online geography and GIS courses, including 'Problem-Solving with GIS', 'Cartography and Visualization', 'Open Web Mapping', and 'Location Intelligence for Business'.