The Spamhaus Block List (SBL) is a realtime database of IP addresses of spam-sources, including known spammers, spam gangs, spam operations and spam support services.

The Spamhaus Exploits Block List (XBL) is a realtime database of IP addresses of illegal 3rd party exploits, including open proxies (HTTP, socks, AnalogX, wingate, etc), worms/viruses with built-in spam engines, and other types of trojan-horse exploits.

From their press release:

"... the Spamhaus Block List was widely adopted in 2003 by major banks, airlines and industries to whom email delivery integrity is vital. In December 2003 the number of mail users protected by the SBL surpassed 200 Million. With the addition of the XBL, Spamhaus aims to provide the maximum spam blocking possible with the safest solution."

More info is available at:

http://www.spamhaus.org/news.lasso?article=151