DSBL, the Distributed Sender Blackhole List, seems to have gone missing. The list appears to have been in operation since at least May, 2002.
The website at www.dsbl.org is not currently responding. Their mail server specified in their MX record is non-responsive, and any DNS mirrors still responding seem to be hosting an out-of-date copy of the blocking list's zone data.
I've talked to the maintainer. The server hosting DSBL died; attempts are underway to find new hosting and a new project maintainer, but it doesn't sound as though anything that would lead to immediate resurrection of the site and data.
Three zones were available: list.dsbl.org, multihop.dsbl.org, and unconfirmed.dsbl.org. At this time, I would recommend that you remove these lists from your mail server configuration, as queries may time out, and any responses returned will be out of date.
March 31, 2009 Update: DSBL is dead. Click here for more information.