Most people do not know that some digital cameras (ones that have RAW shooting mode with more than 8 bit color) allow to squeeze out much more ISO sensitivity than one can set in camera settings. Simply set the camera to its highest ISO setting, set exposure compensation to -1 or -2 steps and shoot in RAW mode, then apply digital exposure compensation by +1 or +2 steps in RAW converter. This allows to utilise extra bits of color representaion recorded by the camera in RAW mode only and simulate 1-2 steps higher ISO speed. This way EOS 300D with hacked firmware squeezes out ISO 12800 (shoot at ISO 3200 and -2 EV correction and then brighten the image in RAW converter). You can even go further and add 1-2 more steps of actual ISO speed in image editing software (after RAW conversion), but then you have to set exposure manually because most cameras can compensate for 2 EV max.
I'd call this method "digital pushing". Of course it greatly increases noise in images, something like increase of grain when a film is pushed to higher sensitivity chemically. But you may want to use this method when you need to take a picture using higher shutter speed with low available lighting, no matter the cost, or to add "arty noise" effect. Also pushed images look better after conversion to black-and-white because the terrible color noise is somewhat really ugly on most images.
Here is a photo of myself taken at ISO 12800 via Canon EOS 300D:
More examples can be found at Academ.org forum. ISO 12800: original image, black and white, 1:1 crop and ISO 6400: 1:1 crop.