Bignum

by Hobnob
Integer bignum storage and arithmetic
ID:112188
 
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.
http://www.byond.com/developer/Asielen/RealBignum