[The Killing-Hopf theorem][1] (yes, Hopf is already dead).

[The ugly duckling theorem.][2]

[The no free lunch theorem.][3]

[1]: http://en.wikipedia.org/wiki/Killing-Hopf_theorem
[2]: http://en.wikipedia.org/wiki/Ugly_duckling_theorem
[3]: http://en.wikipedia.org/wiki/No_free_lunch_theorem