The On-Line Encyclopedia of Integer Sequences (OEIS)

    The largest online integer sequences database, searchable.

    Started in 1967 by N.J.A. Sloane, then edited as two books, now is the online version, now featuring more than a quarter of a million sequences.

    You also have a video illustration of 1000 of these, illustrated by a sequence converted into notes:

    Free, No registration, No Download/Install required