DroidCam is a free program that will let you use an Android phone as a wireless webcam.
DroidCam supports various video formats and resolutions. Format A is the original format used by DroidCam. It uses less CPU but a lot of network resources. Unless you have a perfect WiFi connection, it might not be very smooth. Resolutions over 400×300 are difficult and are disabled. At home, it should work great for most users and will drain less battery.
Format B for advanced video encoding uses minimal network resources (about 100x less than Format A!). This comes at the cost of high CPU usage. There is also a delay due to video buffering. Use this for slower connections.
Format C uses some CPU and about 10x less network than Format A. If you are running Android 2.1 you cannot use this format.