Use your android phone as webcam using USB.

You can use your android as webcam even without wifi by connecting it to your PC via USB. But doing this requires extra work because android can communicate with PC via USB only in debugging mode by using adb.

Also, for USB mode, we need another app called DroidCam. Although the earlier IP Webcam app is more popular, it is more or less limited to WiFi connections especially for windows. So lets see how to  configure your android as a webcam via USB -

Setup your phone in debugging mode (Settings -> Applications -> Development -> USB debugging).

Connect the phone to your computer via USB (don't select storage mode if the phone asks while connecting USB).

Download DroidCam from android market,install it and open it on your phone. It will show a "Starting server" message.

Download and install the client application in your computer from Dev47Apps.

Start the DroidCam client and choose the "USB" option or button.

That's it. DroidCam will now open the camera on your phone and you can use it as webcam on your PC. Just configure the video settings of your video call application to use "DroidCam" driver. Here's my Google+ hangout with video from my android phone connected via USB.

For Linux, you can use the IP Webcam app as well. Just connect your phone via USB in debugging mode to your computer and start the driver script. It will detect and use adb to initiate a connection using USB automatically