I collected Hot Wheels from 1995 up until 2007, I had one room in our house with 3D wall paper, all Hot Wheels. 2,600+ cars and one thumbtack for each one too. When we decided to sell the house I packed the cars up and was tempted to sell them. I got an offer and sold them.
I did keep all of my MLB Promotional cars (44 of them) all packaged still. Those are pretty special to me since I like following baseball and how the cars were only available at one game and not mass-produced or sold in stores.
I also keep an eye out for new '70 Chevelle releases and variations, I have a few clear-plex display cases with all Loose '70 Chevelles. as of Jan.2012 I'm up to 102 of the '70 Chevelles (2 are Code3). I believe the only one I'm missing is the 1999 Employee Birthday version.