Later Edit: ProxyBL is no longer available.
ProxyBL.org is a new service that lists available open proxies on the web. The service is meant to serve administrators that need to block suspect/dangerous IPs, but it can also be used to achieve online anonymity.
What is ProxyBL?
The service crawls the web for IP addresses of possible open (ie publicly accessible) proxy servers. These IP addresses are then scanned and added to the database, classed according to the result (open proxy/closed proxy/no proxy/unreachable/etc).
The interest in open proxies is high due to the fact that these proxies can be used trojans/bots/spammers/kiddies/etc. to conceal their true host.
Free for both commercial and non-commercial
This service offers the data in the form of a DNSBL and can be used freely by both commercial and non-commercial applications. If you are interested you can find more details about the usage here.
Also, you can use the service to check an IP address to see whether it is blacklisted or not.