IP Camera Viewer is a free tool which allows you to view live video from IP or USB cameras.
It's easy to get started. Click "Add Camera", choose your camera type (there's support for 2,000+ IP cameras and webcams should work right away), and click OK to accept the default settings. The video feed appears right away, and you can zoom in, pan around or view it full-screen.
Click Manage Camera > Edit Camera and you're able to tweak various settings, including video resolution, frame rate, microphone level, brightness, contrast and more.
What makes IP Camera Viewer interesting is you can add up to four cameras, then control and view them all simultaneously. You can define how your cameras are arranged, and it's possible to display their views on separate monitors.
IP Camera Viewer is only a viewer - there's no motion detection, recording or anything else - but it's easy to use and the multi-camera support is a plus.