Internet Explorer Mobile 6 provides MUCH improved rendering and web site support for Windows Mobile. The package located here includes Windows Mobile 6.1.4 device emulator images (Professional and Standard) that can be used for development and testing with Internet Explorer Mobile 6.
Internet Explorer Mobile 6 has a nice feature to let you determine whether it renders as a mobile browser or as a desktop computer by changing which user agent string is passed to the web site:
You can change it on options (above left) or under Menu | View | Mobile or Desktop (above right). When accessing msn.com in mobile mode, you still get the mobile.msn.com page as before.
In desktop mode the page renders normally (below left) albeit you will need to zoom and pan (below right) like you have to with other desktop-like mobile browsers like Mobile Opera or Safari:
For developers, the rendering engine is from Internet Explorer 6 on the PC, however the script and Ajax support is based on JScript 5.7. To test things out, I browsed the online ASP.NET AJAX Control Toolkit page at this link on the device emulator in desktop mode and the samples I tried worked great: