This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
The main uses for the OEIS are to identify a sequence, and to obtain more terms, formulas, references, links, programs, etc. for a sequence.
You can search for integer sequences by entering the first few terms, or some words to describe it.
Leave off the first term or two, because people may disagree about where the sequence begins.