OR-Library is a collection of test data sets for a variety of OR problems.
A full list of the test data sets available in OR-Library can be found here.
OR-Library does not have any test problems for the maximum clique problem.
OTHER SOURCES
A variety of test problems are available from DIMACS, via
anonymous ftp here
(see also here)
Test problems are also available here.
A test problem generator can be obtained from Professor P M Pardalos
(email: pardalos@math.ufl.edu)