TurnTide was a company that spun out of the ePrivacy group. Funded by angel investors including the founder of Half.com (acquired by eBay for $300M). Their product was the first "anti spam router", a network appliance that used a dynamic traffic shaping approach to slow spammers down -- rather than block them entirely.
Symantec bought TurnTide about a month after acquiring Brightmail (for $28M in cash). Some reports indicate that the acquisition of TurnTide was intended to provide Brightmail customers with a solution to the embarassing scalability problems generated by Brightmail's sophisticated content filtering algorithms. Since then the TurnTide technology has disappeared inside Symantec's new anti spam appliance product, where it resides alongside Brightmail filtering technology.
Other companies have since taken notice and have begun integrating traffic shaping technology into their anti spam offerings. For example, IronPort now dynamically throttles back sources that have a poor reputation in IronPort's reputation database.