SmallGroups by E. A. O'Brien, B. Eick, and H. U. Besche Small Groups Library. All groups of order up to 2000 (except 1024) and some infinite series of groups characterised by the prime number decomposition of their orders.
Transitive Permutation Groups by A. Hulpke Transitive permutation groups of degree up to 30.
Primitive Permutation Groups by C. Roney-Dougal and many other people. All primitive permutation groups of degree <2500.
Tables of Marks by G. Pfeiffer and T. Merkwitz Tables of marks for many almost simple groups.