Stack Overflow is a knowledge-sharing platform for developers, founded in 2008 by Jeff Atwood and Joel Spolsky. It assists programmers in solving coding challenges, exchanging information privately, and finding job opportunities.

Upwork is an online work marketplace that connects businesses with independent talents worldwide. It offers a platform that enables companies and freelancers to collaborate and unlock their potential across various industries and skills.

Quora is a question-and-answer website that enables people from around the world to share and learn across a wide range of interests. It was founded in 2009 and is based in Mountain View, California, United States.