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.
%I A009561 #5 Mar 30 2012 18:35:07 %S A009561 1,4,13,28,61,119,226,423,790,1472,2764,5224,9956,19112,36923,71703, %T A009561 139835,273625,536860,1055588,2079066,4100573,8096789,16002500, %U A009561 31652194,62648225,124068499,245826760 %N A009561 Index of central binomial coefficient C(2n,n) within A006987. %K A009561 nonn %O A009561 2,2 %A A009561 _David W. Wilson_