I've always heard that Mendeleev came up with the periodic table in a dream, but I haven't found the source of this story.
That having been said, I've started using dreams to work on problems I'm dealing with. Whenever I'm stuck and getting tired, I'll lay down, close my eyes, and enter a dream like state while still focusing all of my attention on the problem I'm working on.
When it ends up working, I'll fall asleep with my mind still working on the problem. Granted, since I am in a dream state there can be a lot of nonsense that my mind will come up with, but it will also make interesting associations I might not have thought of when in my right mind.
Dreams are also one of my primary sources of poetic inspiration, and I've described many of my dream experiences in the form of poetry--something belonging to the tradition I practice.
What do you use dreams for?