jsMessage is a small (3kb gzipped) JavaScript library for displaying stylish alert & notifications messaegs.
Besides notifications, it supports alerts + confirmations and doesn’t require any other JS libraries to run.
The positions of the messages, their auto-expire duration and whether they can be controlled via keyboard or not can all be configured.

Each message type (except notifications) has callbacks for a customized interaction and the look/feel of messages can be completely styled.

This library was extracted from the code of DHTMLX suite. jsMessages library can be used under terms of MIT license (basically free).
DHTMLX is a JavaScript library that provides essential functionality for building cross-browser, Ajax-based user interfaces. Develop impressive web applications faster with a set of ready-to-use UI widgets.
Requirements: No Requirements
Compatibility: All Major Browsers (except IE6)
Website: http://dhtmlx.github.com/message/
Download: https://github.com/DHTMLX/message