Plenty of programs do. In fact almost all. But many charge much more than 50% of return award cost for one-way, thus not good use of miles in most situations.

Off the top of my head some that charge 50% of return cost for one way include Qantas, Virgin Blue and Air NZ.

In the *A forum, the sticky reference links has a link to a thread on one-way awards for all *A airlines (do they allow and if so at what cost compared with return?).