Irrlicht Engine Wiki,
Irrlicht Engine seems
to have a fair number of people using it. The games are realistic though
you seem to be able to walk through people.
I tried
DungeonTech,
which seemed ok. It also looked like the DungeonTech code was available,
and the person in charge seems to be in Surrey.
There were some other games.
I think you have to add the physics from another source.
Level editors seem like a useful tool. Irrlicht doesn't seem to have them,
Quake does, Delta3D seems to.