big.js
A small, fast, easy-to-use library for arbitrary-precision decimal arithmetic
bignumber.js
A library for arbitrary-precision decimal and non-decimal arithmetic
decimal.js
An arbitrary-precision Decimal type for JavaScript.
big-integer
An arbitrary length integer library for Javascript
jsbn
The jsbn library is a fast, portable implementation of large-number math in pure JavaScript, enabling public-key crypto and other applications on desktop and mobile browsers.