I've caught a few domain names over the past couple of years and been in auctions for some I didn't get. To find out what's dropping, I use justdropped. There are things I like about it, but there are some things that frustrate me. For one thing, the searches I perform never show .orgs that are dropping. Another limitation is that there is no way of finding names that have PR or inbound links. are there any other services out there that do this?