(the site is mostly of historical value by now, but should be an interesting read)
Zaptel has been renamed to "DAHDI", but still contains the full specs of the Tormenta and Tormenta-II card design and firmwares. Sadly it has not been carried on as a free hardware project, and newer Zaptel hardware are forks of it (e.g. "tor3") or complete redesigns.
(For the curious: "Zaptel" had to be renamed because http://zaptel.com/ sells telephony cards of a very different nature)