In the Law & Order verse: Lot one is Jack, Claire, and Mike, with Mike sometimes shuttling back and forth between Ben Stone's house, and his own. >< Oy vey. Lot two is Munch and Lennie, and they're currently snarking and no longer in love. Talk to Lennie, Munch, there ya go...and Ed Green and Odafin Tutuola are sharing a house with Elliot and Olivia, with Goren sleeping over when he feels like it. It really is a soap opera.
West Wing: Everybody's sleeping with everybody. Least complicated version there is. ;)
LoTR: Sam and Frodo have moved in with Merry and Pippen, and Legolas and Aragorn are hanging out with Arwen and Eowyn. Sheesh.
There are a few more random fandoms (Quantum Leap and MASH, anyone?) but these are the ones I dabble in the most.
Hee. I love playing God.