Official url is http://www.math.leidenuniv.nl/~buytenh/bridge/. With developer kernel 2.3.47 the new bridging code is part of the mainstream.
The Bridge-Mailinglist is homed at http://www.math.leidenuniv.nl/mailman/listinfo/bridge.
This document has it's official homepage at http://www.bnhof.de/~uwe/bridge-stp-howto/BRIDGE-STP-HOWTO/. It's a part of the Linux Documentation Project located at http://www.linuxdoc.org/.
Download Types and Locations
http://www.bnhof.de/~uwe/bridge-stp-howto/BRIDGE-STP-HOWTO.tar.gz
http://www.bnhof.de/~uwe/bridge-stp-howto/BRIDGE-STP-HOWTO.html.tar.gz
http://www.bnhof.de/~uwe/bridge-stp-howto/BRIDGE-STP-HOWTO.pdf.gz
http://www.bnhof.de/~uwe/bridge-stp-howto/BRIDGE-STP-HOWTO.ps.gz