I of course do much of the "work on A, reference on B" that others have talked about--the IDE open on one screen and the documentation open on the other--but it's also worth pointing out the cases where there are multiple pieces of reference material that I'm trying to collide somehow, and having both of them open simultaneously is obviously incredible useful.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one. (Immediately before; refresh the list-of-threads page before posting.)
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.