The following are a few of the tools I use for testing mobile sites on Windows 7, prior to testing on actual devices:

Opera Mobile Emulator
Emulates the behavior of the Opera browser under multiple devices such as various HTC, LG, Motorola, Nokia, Samsung, Sony Tablet, Amazon Kindle, and a few others. This is my testing tool of choice because it is easy to use, fast and a close representation of the mobile experience.

Safari for Windows
Since Safari is the actual browser on Apple's devices, Safari for Windows is a good option to test the rendering engine. You can also set the User Agent to iPhone, iPad, IE, Firefox and Opera from the Develop menu (activate the Develop menu from Settings → Preferences → Advanced.)

Reducing the screen size to match the desired mobile screen size completes the experience. I found the best way to resize the browser in Windows is to download the L-Square Safari extension to get a very useful pixel ruler:
Visit from Safari. Choose the Developer category and install L-Square. This will add the ruler button in the toolbar next to the address.

More info:

Android Emulator SDK

This is a full-fledged Android emulator for Windows. It takes a couple of minutes to get it going, but you can test on an actual instance of mobile Android, complete with a slow browser and on-screen keyboard. Advanced users can install additional Android browsers such as Fennec: