Good Service for Being Notified of Expired Domains?

I want to track about 500 domains…is there a good service that will notify me when any of these become available.

It would be cool to do this with a Google Alert.

You might be able to get a programmer to make an alert program for you. It would cost $xxx to $xxxx but once complete, you could use it on a website and charge a fee for other people. I know GoDaddy has a backorder service as do some of the other players in the industry but for 500, that could get expensive.

I have never used such services, But I suppose you can do some search for this on the namepros forums.