When it comes to testing responsive webdesigns, testing with real mobile devices can hardly be replaced with anything. However, testing alpha versions of your web project can be done easier. I've read an article about Opera Mobile Emulator today on Smashing Magazine, and it seems to be quite useful. It's Opera Mobile available for regular computers, allowing for checking how a website looks and behaves at different resolutions and pixel densities.
You can download Opera Mobile Emulator from the Opera website. Setting it up on my Ubuntu system was as easy as installing the provided DEB file.
When starting the Emulator, you can choose from a number of mobile devices, each one pre-configured with its specific resolution and pixel density. You can add more devices yourself. After clicking the "Launch" button, the Opera Mobile browser gets started with the chosen settings.
The application reacts very fast, much faster than using the emulator shipped with the Android SDK. It uses the original rendering engine used on the mobile devices.