Lenstra's LIP package
    Arjen Lenstra   Bellcore
    
    Portable unsigned integer package written entirely in C.
    Includes +, -, *, /, exponentiation, mod, primality testing,
    sqrt, random number generator, and a few others.  
    An earlier version of this package is the only of these packages
    I have actually used.  It works well and is very portable.  
    I haven't done any benchmarks against the others, but the code 
    looks clever & Lenstra is an accomplished number theorist.

    LIP replaces lenstra-3.1.c.  The package now includes encrypted
    source code; to obtain the decryption key, you must send a
    signed license agreement to Bellcore.  See the documentation.