ShadowPlay, planned for Summer 2013, constantly works in the background utilizing the H.264 encoder built into every Kepler GPU from NVIDIA to auto-record your last 20 minutes of gameplay.
Having been a longtime Fraps user, I am still not impressed by this program's inception assuming recording cuts off at the 20 minute mark. Fraps is CPU intensive, though the problem of lagging while recording something can be alleviated by writing the video data to a separate HDD, convenient for RAID setups.
Also, Fraps record each frame of the video uncompressed, which allows me to do whatever I want with it using a 3rd-party video editing tool. Nvidia's compression methods are still not clear and I am therefore skeptical of the quality of the footage it will produce.