At Newzbin we used them for reducing load on MySQL during Usenet header fetching - each new header would have the Message-ID put through a filtering service to

At Newzbin we used them for reducing load on MySQL during Usenet header fetching - each new header would have the Message-ID put through a filtering service to check if it had already been inserted. The service kept an array of 7 filters, rotated daily - the oldest would be cleared and reused for new items, giving us 6-7 days of history. Each individual header was low-value, and a few false positives every week wasn't a big deal - Usenet servers lost a lot more during their normal course of operation.