Decimal.js
An arbitrary-precision Decimal type for JavaScript
Tiny Color
Fast, small color manipulation and conversion for JavaScript
Auto Numeric
autoNumeric is a jQuery plugin that automatically formats currency (money) and numbers as you type on form inputs. It supports most International numeric formats and currency signs including those used in Europe, North and South America, Asia and India (lakhs**).
Mathjs
An extensive math library for JavaScript and Node.js
Poly
Small, fast, awesome. The only ES5-ish set of polyfills (shims) you can mix-and-match because they're individual AMD modules.
Sprintf.js
sprintf.js is a complete open source JavaScript sprintf implementation.
Babel
Babel is a compiler for writing next generation JavaScript.
Store
A better way to use localStorage and sessionStorage
Typecast.js
Typecast.js :: Solving the simple problems
Numeral
A javascript library for formatting and manipulating numbers.
Ratio.js
Rational numbers for Javascript
Css3 Finalize
Tired of adding prefix for each broswer when it comes to the new css3 attributes?
Placeholders.js
A JavaScript polyfill for the HTML5 placeholder attribute
Object.observe
Object.observe polyfill/shim trying to be compliant
Nerdamer
a symbolic math expression evaluator for javascript
Args.js
Create javascript functions with optional, default, grouped and named parameters.
Core Lib
A port of the Ruby 1.9.3 corelib to coffeescript/javascript/node that conforms to rubyspec.org.