One of the things I've started to (stupidly) hang onto are some of the battlepack "backgrounds". I haven't used any of them for anything, but I guess I think I will someday. That might be something that should just be tossed out though. I have kept some things from vehicles as well (Zev's Snowspeeder, Dagobah X-Wing, etc.), and some of those don't look too bad.
I'm holding on to the backgrounds from ships, BPs, Cinema Scenes, etc. as well, but I should probably just toss them. I've used a few for diorammas, but 95% of them are collecting dust.