I purchased a couple Ataris and a bunch of games at a garage sale on a whim with designs to resell it all. I kept one of the systems, and have since been collecting cheap Atari stuff as I find it.
My parents had an Atari when I was young, and it was one of my first exposures to video games, so it also has a bit of a nostalgic factor for me.
I try to collect versions of games that have nice labels, as I think the art is about half the fun of owning a game. They also have to work, obviously.
Here's my collection so far (click on pictures to enlarge)