I have used Expired Domain Sleuth and it has worked ok for me. The real kicker with the software though is you either need a list of working web proxies or pay for the monthly membership option as your ip can get banned from search results if you are examining many expired domains.
The other tool I could suggest is a website:
DropScout.com
The nice thing with this site is that it list the
PR of the sites that are expiring - though if you want one with really high
PR, good luck. It seems you need to get ahead of the rest of the world if you want to catch one of those.
Lastly, you can also try the Domain Name Aftermarket from GoDaddy.
Tdam.com
The neat thing here is you can pick up expired domains that have not dropped - which is better as the age of a domain tends to affect the search engine ranking.
Hope that helps you out.