The On-line Encyclopedia of Integer SequencesNeil J. A. Sloane, AT&T Fellow
This encyclopedia answers the question "What comes next after 1, 2, 4, 9, 20, 48, 115, 286, 719, ... ? (for example). The main table is a collection of number sequences arranged in lexicographic order. The entry for each sequence gives the beginning of the sequence, its name or description, any references or links, any formulae cross-references to other sequences the name of the person who submitted it, etc.