I refer to these sites when I have downtime at work or when I’m bored. I’ve managed to pick up a few new skills (for work and for fun). Here are the sites and a quick word on what I use them for.
OpenCourser. Good for finding an online course to take, more for in-depth learning + building professional skills.
NoExcuseList. Great for discovering new learning websites. Neatly categorized into sections (like cooking, music).
KhanAcademy. (Re-)Learn pretty much every subject taught in K-12. Edit: Also great for college-level math and exam prep (SAT, GMAT, MCAT, etc).
StackExchange. Get answers from people who are super knowledgeable about all sorts of different things.