The HTML5 Shiv : The HTML5 Shiv enables use of HTML5 sectioning elements in legacy Internet Explorer and provides basic HTML5 styling for Internet Explorer 6-9, Safari...
SlimerJS : A scriptable browser for Web developers
WebTorrent : WebTorrent is a streaming torrent client for the web browser and the desktop.
WebTorrent is written completely in JavaScript – the language of the web...
LokiJS : A fast, in-memory document-oriented datastore for node.js, browser and cordova
A JavaScript file you can insert into your pages to simulate the Firebug console in browsers that are not named Firefox.