PHP bcmath as a class, with several enhancements.
Extends php BCMath lib for missing functions like floor, ceil, round, abs, min, max, rand for big numbers. Also wraps existing BCMath functions.
Measure classes to facilitate storing and converting units of measure. Uses bcmath functions for arbitrary precision.
AzaMath - Anizoptera CMF mathematic component. Provides functionality to work with large numbers with arbitrary precision (using BCMath) and universal mumeral systems conversions (supported bases from 2 to 62 inclusive, and custom systems; pure PHP, but can use GMP and core functions for speed optimization).