VMatrixLib is a little, handy, Java matrix library which implements a set of utility functions (such as calculus of the determinant of a matrix, calculus of the rank, inversematrix, and many others..) that anyone who have to use matrices needs.
Slate++ is a vector/matrix package for C++. Based on templates, Slate++ facilitates basic linear algebra (+, -, *, /, inverse, determinant, ...) on many data types (int, float, double, complex...). Simple to use and thoroughly documented.