The JTS Topology Suite is an API for modelling and manipulating 2-dimensional linear geometry. It provides numerous geometric predicates and functions. JTS conforms to the Simple Features Specification for SQL published by the Open GIS Consortium.
x86mph is an optimized library with vector, matrix, and vertex helpers (including an own TnL) It also contains memory helpers, and other stuff such as image processing (color inverting, changing bpp) taking advantage of x86's 3DNow!, MMX, and SSE