This is my take on them, and why I decided to try to make my own.
|date.js||Great, but only handles dates. Hasn't been updated since 2008.|
|jsTimezoneDetect||Useful utility to detect the browser's time zone.|
|GWT||i18n Guide - lots of support, but only useful if you are using GWT, and yet it is very different from the standard Java libraries. And each additional locale lengthens compile times.|
|Closure Library||goog.i18n - only a couple of languages, coupled to Closure|
|dojo||dojo.i18n - Comprehensive, but tightly coupled to the rest of dojo.|
|jquery.global||from Microsoft and uses MS terminology (Cultures vs Locales). Cryptic (and limited) format codes for dates and numbers.|