If you want to use two brains, 'partner mode' for the New Yorker's crossword has been great. Two persons can work on a single puzzle in real time. There may be

If you want to use two brains, 'partner mode' for the New Yorker's crossword has been great. Two persons can work on a single puzzle in real time. There may be other sites that allow it -- the software seems to power a few diff crossword sites (with certain features enabled/disabled).