When G is a locally
compact group, the unitary representation theory of G is the “same” as the
∗-representation theory of the group C∗-algebra C∗(G). Hence it is of interest to
determine the isomorphism class of C∗(G) for a wide variety of groups G. Using
methods suggested by papers of Z’ep and Delaroche, we determine explicitly the
C∗-algebras of the “ax + b” groups over all nondiscrete locally compact fields and of a
number of two-step solvable Lie groups. Only finitely many C∗-algebras arise as the
group C∗-algebras of 3-dimensional simply connected Lie groups, and we
characterize many of them. We also discuss the C∗-algebras of unipotent p-adic
groups.