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I think the most obvious alternative for data centers is a natural gas backup generator. Start time is longer than with diesel though, so you have to take that into account. There is also the issue of potential interruption of natural gas supply in the event of some natural disasters, such as earthquake. This brings up the option of duel fuel generators. Honestly though, there is relatively little time that backup generators are running, even with regular testing, so I'm not sure how much difference it makes in the big scheme of things. More efficient cooling would probably be a much greener investment, and potential money saver.
The link supplied by Jim Lynch mentions the main alternatives -- solar, wind and water. Another site called Eco Friendly Green Earth writes about magnetic generators.
Then there's "pedal power," but that's probably not a viable solution for an enterprise.