Reusing computer system's heat - Grey Energy?

Computer systems generate a lot of heat. There is great effort to remove that heat using passive or active systems (fans, pipes, etc). Then there is now efforts to design cooler systems. I wonder if this excess heat can also be reused. Somewhat like "grey water" systems reuse some types of water in a home. The amount available is huge. Imagine reusing the heat energy from Google server farms. I won't state that it can; invariably, such statements tend to be demolished after simple analysis, for example, the cost to capture and reuse the heat energy may be too high or have a low ratio. Anyway, just a thought. --- Josef