Jürgen Moser (1965), On the Volume Elements on a Manifold , Transactions of the American Mathematical Society, Vol. 120, No. 2 (Nov., 1965), pp. 286-294
http://www.jstor.org/stable/1994022
Besides the many powerful applications of the famous "Moser argument" (or "Moser trick"), the local version gives a very nice and elegant proof of the classical Darboux Theorem.
(For a nice summary of this and other papers by Jürgen Moser, I would recommend http://www.math.psu.edu/katok_a/pub/Moserhistory.pdf (a short discussion of the paper mentioned above can be found at p.17-18))
