Still the best mod is Unified Zombie Mod from Scott Tunstall even if it's dead for a long time now.
Last release was Unified Zombie Mod Beta 3 available at Farmland Mod:
http://ofp.gamezone.cz/_hosted/farmland/down.htmlA FAQ thread is available for it at the BIS Mission Editing forum.
The newer Zombie addons come with there own moan scripts or some custom animantions, but they can't be called mods. No optimization, not well documented, not very customizeable, allmost no publicity and they often consist of an edited Unified Zombie Mod addon.
There are some (maybe 3) coop missions. Most of them even work without addons, as the makers imported the ai scripts into the mission. Search the BIS User Missions forum for "zombie coop".
Singleplayer missions I know are:
Night of the living Conscript (OFPEC)
Doomsday I-IV (OFPEC or BIS forum)
Biohazard Apocalypse (BIS forum. Only a beta with bugs, but worth a try)
Trapped on Malden (BIS forum. A newly released beta campaign.)
Some Zombie missions are also available on the Farmland page.
ZOS Zombie Outbreak Simulation (OFPEC)