TITLE: Reference for tetrahedral Coxeter group
QUESTION [6 upvotes]: Let G be the group with 4 generators, each of order 2, such that the product of any 2, say ab, has order 3 (i.e., ababab=e).  That is, this is an infinite reflection group with Coxeter diagram a tetrahedron.  I am looking for references for this group...

REPLY [3 votes]: The group itself shall be the group generated by reflections in the sides of a regular ideal tetrahedron, whose dihedral angles are all $\pi/3$. For a reference,  there are many Coxeter diagrams listed in a paper by Johnson, Kellerhals, Ratcliffe and Tschantz, 
called "The size of a hyperbolic simplex". 
Please mind the fact that you may have to look at the barycentric subdivision of your simplex before you find its counterpart (a simplex from the subdivision, which is an orthoscheme) in their table. 
Definitely, the ideal simplex reflection group contains interesting subgroups, which are manifold groups. There should be the eight-knot group in there, since the figure-eight complement comes from glueing two regular ideal tetrahedra. I suppose that Neil's references shed more light on this kind of questions. Hope my reply describes in more geometric detail the group you were interested in.