Specialized Hardrock, Boardman Hardtail comp, Scott Aspect. But to be honest, as long as you don't go finding a Fleabay special, you'll be fine. Make sure there's a good frame, the running gear can be upgraded as & when it wears out.
If you aren't choosy about the latest colours & kit, buy a 2010 model & save more money. For a good unbiased price point breakdown, go forth to thy newsagent & purchase What Mountain Bike magazine. There's a great buyers guide section at the back.
Reserve some of your budget for helmet, Camelbak, gloves, shorts (padded cycling shorts are a must).
I recommend finding your Local Bike Shop (LBS) and go ask questions. The support from a good LBS beats the hell out of internet bargains. Plus they should advise you on sizing the frame, adjusting the riding position & so on.
Finally check (if applicable) in case your employer runs the Cycle To Work scheme; it's a great way to get a bike for little outlay.