I use procmail to keep most of the spam flood from my mailbox, as "Protect yourself from spam: A practical guide to procmail" explains.
Scott Hazen Mueller's Fight Spam page, Dougal's AntiSpam Page, Anti-SPAM Tools, Ian Leicht's Nags filter, the Abuse.net home page, the home page for the spamtools mailing list, ...
For more on sendmail-specific filters, see the Anti-Spam Provisions in Sendmail 8.8.