As this is a bass forum the first items that come to mind would be tackle. I myself, collect rare Daiwa and Megabass reels. The reason I collect rare/ limited reels is that I am attracted to the mechanics and technology put into a reel as well as the limited side to the models i'm into. Furthermore, unlike rods, they fit in my display. Why not lures? there is just too many and I feel like it would clutter up a display. I keep my collection separate from the tackle that I use and don't see why some people have it in for those who also enjoy the collecting aspect of tackle ?
Apart from tackle, I caught the collecting bug young and started with stamps. Not something I have spent much time on in a long time, but I should really do something with all those albums of stamps I have stored away.
What about yourselves, what do you collect and for what reason?
Do you collect tackle or do you think tackle is only made to be fished?