by Hobnob
Integer bignum storage and arithmetic
Provides support for the storage and use of 'bignum' integers with an arbitrary number of digits. Procedures for standard arithmetic operations are provided.

Also includes a demo of a 24-digit integer four-function calculator to show how the bignums may be used.
Great library. Should be expanded for decimals though.

Edit: I went ahead and created a new bignum library from the ground up. It supports decimals and also is orders of magnitude faster.