Matrix4.extractBasis, Matrix4.makeBasis - very useful for messing around with orientations.
Matrix4.makeShear - standard shear constructor with optional Autodesk Maya style construction.
Euler.toVector3(), very useful.
add Quaternion.add/sub/multiplyScalar
improved Matrix4 type checking on Euler, Matrix3 and Quaternion for ambiguous cases.
add Euler.setFromVector helper.
Matrix4.multipleList - easy multiplication of an array of Matrix4.  Useful for hierarchies.
address @mrdoob's and @WestLangley concerning new math additions, add unit tests.
remove Matrix4.makeShear per @WestLangley here: https://github.com/mrdoob/three.js/pull/5788#issuecomment-67852744
add docs per @WestLangley's request: https://github.com/mrdoob/three.js/pull/5788#issuecomment-67857995
Merge pull request #5788 from bhouston/math-improvements

Various Math improvements from Clara.io