[[http://www.gap-system.org/Packages/sgl.html|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.

[[http://www.gap-system.org/Datalib/trans.html|Transitive Permutation Groups]] by A. Hulpke
Transitive permutation groups of degree up to 30.
 
[[http://www.gap-system.org/Datalib/prim.html|Primitive Permutation Groups]] by C. Roney-Dougal and many other people.
All primitive permutation groups of degree <2500.

[[http://www.gap-system.org/Datalib/tom.html|Tables of Marks]] by G. Pfeiffer and T. Merkwitz
Tables of marks for many almost simple groups.