There’s an interesting project from Microsoft’s OfficeLabs: Touchless, a project which allows you to utilize a webcam to do multitouch style input on your PC.
The basic idea is something I’ve thought about as well – have a webcam track a colored object, and use that as a “cursor”. The touchless demo is very promising, although at this time it seems it’s a bit difficult to set up to recognize the objects well. It could be just my webcam too; it doesn’t exactly have a very good quality image. However, when set up properly, it does work surprisingly well.
Anyways, I definitely recommend checking out the project. The demos are fun to play with for a bit at least. If you’re familiar with C# or some other .NET framework language, you can even use the source code provided to develop your own multitouch apps. Note that it requires .NET Framework 3.0.
In case the OfficeLabs site is down, you can find the files from their CodePlex project page.
If you’re having problems getting the software to recognize your markers, you probably should try aligning the camera to face down to your desk so it provides a less colorful background. I also had to adjust my camera’s color balance, but that might not be an issue with a better webcam.