May’s Site of the Month is the On-Line Encyclopaedia of Integer Sequences.
The wiki-page is available in over 50 languages.
This site of over a quarter of a million sequences allows you to enter the first integers of any sequence and search for information on that sequence. It has links to famous, puzzle, classic and “hot” sequences, and is very fully referenced. There is a movie of the first 1000 terms of 1000 sequences. The site also invites you to contribute your own sequence.
This post is also available in: Arabic