SpamAssassin includes support for many of the bigger DNSBLs, with optimal scores (or at least, optimal as determined by the GeneticAlgorithm).
Support for these is built-in, and shipped in the default configuration.
All of the free BL services are enabled by default. However, note that if you're using mail-abuse.org, and you're installing SpamAssassin for site-wide use, you must license their services; mail-abuse.org is now only free for personal use.
Note that SpamAssassin still works very well without using these two services, however.
Live accuracy figures for most of the DNSBLs used in SpamAssassin, based on the Oct 2003 mail feed for one user, can be found here: http://taint.org/2003/11/07/184247a_mail.html
Other places to find out about DNS BlackLists / BlockLists: http://moensted.dk/spam/ http://www.declude.com/junkmail/support/ip4r.htm
Blacklists Compared weekly at http://www.sdsc.edu/~jeff/spam/Blacklists_Compared.html Current Blacklist Comparison http://www.sdsc.edu/~jeff/spam/cbc.html