Two years ago must have been about the time that my domain-buying was at its height. I've been getting a lot of emails from my web hosts asking if I want them to renew domains for me.
I've let the lampmagic.(com|org|net) domains go. I had planned to use them to set up some kind of LAMP systems support resource, but I never got enough enthusiasm (or tuits) for it.
Another domain I've let go is a little more galling. Just over two years ago I was sitting in a restaurant with my wife and a friend and we came up with a good idea for a site. We thought that it would be a lot of fun if there was a web site where you could go and try to find old school friends and get in contact with them. I registered the domain i-remember-you.com but (of course) didn't have the time to do anything with it. A few months later Friends Reunited appeared and was very successful.
Note to self: start acting on your ideas