Hypercalc

HyperCalc is a Perl program written by Robert Munafo that is designed to manipulate very large numbers by storing them in a notation called "PT vectors," in which $$a \text{PT} b = \underbrace{10^{10^{10^{.^{.^{.^{b}}}}}}}_a$$. It is licensed under the GNU General Public License, and operates through a terminal interface.

Kenny Chan created an online version by porting the program to JavaScript.