As of Sunday, May 27th, 2007, the blocking list Spambag, with the DNSBL zone blacklist.spambag.org, is no longer available. The website www.spambag.org does not resolve to an IP address, and there appear to be no DNS entries under the DNSBL sub-zone.
Spambag, created and run by Sam Varshavchik, developer of the Courier mail server, has been operating this list since at least November, 2001.
The list had the following listing criteria: "[Spambag is my] personal list of networks who I block from sending me mail or accessing my web servers, because I believe the networks actively or passively allow abusive or antisocial behavior. Examples of what I consider abusive or antisocial behavior are: spamming, mailbombing, mail server dictionary attacks, and web page E-mail address harvesting."
I last noted a hit against this DNSBL on May 26th, at 1:34 am US central time. Note that I was not a user of this list; I simply measure its effectiveness and status, like I do for many other lists.
This post to news.admin.net-abuse.email explains that Sam Varshavchik shut the list down, and that he felt his efforts had not been as productive as he would've liked them to be.
I would recommend removing blacklist.spambag.org from your list of DNSBLs to check, as it is no longer in operation.