With my DP it's computers, gadgets of all kinds and talking about science - he and I can watch a documentary about the shape of the universe and then discuss it into the small hours, for instance - thank god we save our friends from each other in that respect.
SuperGeek of course comes to my aid with all techno-related problems and will actually write bits of code to fix computer problems. He also sets up ridiculous amounts of tech relating to all the TV, DVD, audio etc. systems so that they need 17 remotes to operate them, all for the sake of some minor techie benefit that he's revels in but I'm totally unaware of...