A database of sequences

In the Online Encyclopedia of Integer Sequences ( OEIS ) you can find tons of information on any sequence you can possibly think of.

An example.
A002024 n appears n times.
Starts with: 1,2,2,3,3,3,4,4,4,4,5,5,5,5,5,6, ...
Closed form: a[n] = Floor[1/2 + Sqrt[2n] ]

Amazing.