2. Whitelist/verification filters
A fairly aggressive technique for spam filtering is what I would call the "whitelist plus automated verification" approach. There are several tools that implement a whitelist with verification: TDMA is a popular multi-platform open source tool; ChoiceMail is a commercial tool for Windows; most others seem more preliminary. (See Resources later in this article for links.)
A whitelist filter connects to an MTA and passes mail only from explicitly approved recipients on to the inbox. Other messages generate a special challenge response to the sender. The whitelist filter's response contains some kind of unique code that identifies the original message, such as a hash or sequential ID. This challenge message contains instructions for the sender to reply in order to be added to the whitelist (the response message must contain the code generated by the whitelist filter). Almost all spam messages contain forged return address information, so the challenge usually does not even arrive anywhere; but even those spammers who provide usable return addresses are unlikely to respond to a challenge. When a legitimate sender answers a challenge, her/his address is added to the whitelist so that any future messages from the same address are passed through automatically.
Although I have not used any of these tools more than experimentally myself, I would expect whitelist/verification filters to be very nearly 100% effective in blocking spam messages. It is conceivable that spammers will start adding challenge responses to their systems, but this could be countered by making challenges slightly more sophisticated (for example, by requiring small human modification to a code). Spammers who respond, moreover, make themselves more easily traceable for people seeking legal remedies against them.
The problem with whitelist/verification filters is the extra burden they place on legitimate senders. Inasmuch as some correspondents may fail to respond to challenges -- for any reason -- this makes for a type of false positive. In the best case, a slight extra effort is required for legitimate senders. But senders who have unreliable ISPs, picky firewalls, multiple e-mail addresses, non-native understanding of English (or whatever language the challenge is written in), or who simply overlook or cannot be bothered with challenges, may not have their legitimate messages delivered. Moreover, sometimes legitimate "correspondents" are not people at all, but automated response systems with no capability of challenge response. Whitelist/verification filters are likely to require extra efforts to deal with mailing-list signups, online purchases, Web site registrations, and other "robot correspondences".