It's been suggested that I discuss the applications of arbitrary precission integer arithematic, so (to quote from my newly edited "Main" page)...
Encryption is, by far, the most common application of libraries such as this. In SSH v1, for example, there are 768 bit (232 decimal digits) and 1024 bit (309 decimal digits) modulo's that need one needs to contend with when performing modular exponentiation for the purposes of RSA encryption. Since the base number can be up to one less than the modulo, there is a 50% chance that this number will be atleast half as large as the modulo (eg. 512+ bits). Mathematical operations on numbers this large require libraries capable of supporting integers of any arbitrary size, hence this library.
If anyone has any questions or comments or anything, feel free to post them :)