ATLAS of Finite Group Representations - Representations of many finite simple groups and related groups such as covering groups and automorphism groups of simple groups. Available by FTP in alternative formats: Meataxe ASCII, Meataxe binary, and GAP.Character Tables for Chemically Important Point Groups - By Achim Gelessus, Mainz.
Groups of Small Order - A list of isomorphism classes and further information on groups of order up to 30, by John Pedersen, University of South Florida.
The Crystallographic Groups Catalog - A catalog of crystallographic groups of dimensions 2, 3, and 4 which covers most of the data contained in the book ``Crystallographic groups of four-dimensional space''.
Numbers of Small Groups - The good people at GAP, in particular Hans-Ulrich Besche and Bettina Eick have produced an explicit GAP catalogue of all the groups of order up to 1000 (with the exception of the orders 512 and 768). This table simply gives the number of groups contained in their catalogue, so you can quickly figure out how difficult your task is likely to be.
Collapsed Adjacency Matrices, Character Tables and Ramanujan Graphs - A database of character tables of endomorphism rings.
