  We modify the notion of a Fra\"iss\'e class and show that various interesting
classes of groups, notably the class of nonabelian limit groups and the class
of finitely generated elementary free groups, admit Fra\"iss\'e limits.
  Furthermore, we rediscover Lyndon's $\Z[t]$-exponential completions of
countable torsion-free CSA groups, as Fra\"iss\'e limits with respect to
extensions of centralizers.
  Dedicated to the memory of Charles Sims.
