This question has been more fun to think about than I first thought it would be; thanks for the ask.
The easy answer:
1. Google for utility (and all subdomains like inbox.google; drive.google)
2. Wikipedia for knowledge (and Wikimedia)
3. Reddit for community and learning new things both useful and entertaining
4. Quora for the same reasons as Reddit
5. Amazon for supplies of all sorts
The more in-depth and less obvious answer:
1. xkcd and the associated What If? (and also explain xkcdfor when you don’t get the jokes) because Randall Munroe, the author, is hilarious and brilliant, and reading this makes you smarter and happier (if you’ve never heard of XKCD, that makes you one of the Ten Thousand!).
2. Goodreadsis great for discovering new books, tracking your reading, and following other readers and writers. Professionals use it too!
3. Your local school or city library website – this is a great and yet under-utilized resource for students and civilians alike, and can help you with research, fact checking, and just finding something new to be interested in. Nowadays, city libraries are doing more and more to get involved with their communities by hosting events, conventions, and panels.
4.
http://www.sheppardsoftware.com/is how I study geography when the government decides to test me on it. It’s hardly modern-looking, and too educational to be actual fun, but the site has great utility for learning things.
5.
http://www.instructables.com/because it teaches you how to do things, encourages you to do new things, and promotes sharing knowledge and skills.
Honorable mentions:
1. YouTube, Pandora, Imgur and the like
2. EasyBib: The Free Automatic Bibliography Composer which lets you create style-appropriate citations for professional writing
3. LinkedIn because it allows you to network virtually, though it is fraught with security and social faux-pas concerns
4. The blog of your choice – I’m personally into writers and writing advice, as well as sci-fi news and such. My choice here is io9 - We come from the future..
5. Project Guttenberg for literacy
