The Adaptec GameBridge AVC-1400 is a device touted as a way to save your game replays to your computer, but because it's essentially a device to capture video and port it to the PC, it can be used for a variety of different things including saving video from your DVR, DVD player, or even VHS.
The GameBridge consists of two cables and a decoder in the middle. The cables take RCA (red, yellow, white) or S-Video as input, and connect to your computer via USB 2.0. Included is software that can save the video to your hard drive, but it is somewhat old and frequently crashes. The nice thing is you can use dozens of other programs to capture the video, free or not, and it performs much better. MediaPortal and BeyondTV are two such programs with great features.
With software DVR programs like BeyondTV you could even set up a DVR on your computer if you do not have a DVR, but it requires some work. It should be noted that the GameBridge has firmware inside of it to convert the signal from the TV, and as such it will take about 1-2 seconds for it to be visible on your PC, so it is definitely not something you could directly hook a gaming console to and play it on your PC. The length of the cable is about 6 feet, but since it is USB if you need a longer cable, you can purchase one cheaply or you may even have one laying around already.
The best use of this is probably in backing up DVDs or VHS tapes and it's cheaper than TV tuners which offer the same functionality.