I've been using HtmlUnit (Java). This was originally designed for unit testing pages. It's not perfect javascript, but it hasn't failed me in my limited usage. According to the site, it can run the following JS frameworks to a reasonable degree:
- jQuery 1.2.6
- MochiKit 1.4.1
- GWT 2.0.0
- Sarissa 0.9.9.3
- MooTools 1.2.1
- Prototype 1.6.0
- Ext JS 2.2
- Dojo 1.0.2
- YUI 2.3.0
