In math.SE as well as in codegolf.SE we quite often use OEIS (Online Encyclopedia of Integer Sequences), which the standard reference for sequences in combinatorics and number theory.
I think - and apparently I am not the only one - this would be an excellent addition of sites that do get oneboxed.
All the sequences available in OEIS are enumerated and have a consistent formatting, which would make it easy from a technical standpoint. As an example: http://oeis.org/A000001 the "first" sequence, which is the the number of groups (a certain algebraic structure) of the given order.
I picture the onebox to contain the number and the description, as well as the first three lines (there are usually at most three lines displayed there, if more are know they can be found under the list link) including the links that follow, basically just the header as you can see it here (screenshot):