I collect books, nothing else. My wife likes to have lots of books around as well so that's all good. She's not a collector though, but she does come from a family that finds it hard to get rid of stuff, haha... so she has a bit of that tendency as well.
But generally speaking I'm leaning more towards owning less stuff as I get older (spoke the elderly 42-year old). On the other hand I can see myself getting into collecting music and movies as well as books... if I ever get enough storage space.