Ex modder, Zones only ferret and will someday release a game (Resident)
On forum: 02/20/2010
Companions are embedded into several larger mods. The Faction War (TFW) has a companion system included into it, and is complete.
The Zones Call is still a work in progress, but will have companions included into it.
There are several mercenary mods in which you can hire a squad to follow you around. I think one of them is standalone, but I don't know the name of it.
Sadly, any sort of dedicated companion mod for the vanilla game will require a new game. I was building a companion feature for another mod which gave you companions at specific parts of the storyline, and would have worked standalone, but I no longer have the files for it.