Stack Exchange network consists of 175 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.
Some Stack Exchange sites have a 'sponsored by' logo just under the top bar:
Which sites in the network are/were sponsored, by which company/organization/product and when did that start?
Currently, three or four sites in the network are sponsored:
stackexchange.com "sponsored by"