VectorMath:  A fast, free library for 3D Math

During my time at Sony I used VectorMath as my primary math library.  Its a clean implementation entirely in header files.  It also has neon and sse versions should those be useful.

Although developed by SCEE this library is open source and can be used by anyone.

Probably the easiest way is to get it is from the Bullet source code.  Bullet is an opensource physics engine, so if you need one of those it might be worth a look too.