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: https://www.youtube.com/watch?v=LCWglXljevY
Free, No registration, No Download/Install required