www.windguru.com
Once you've registered (free), you can make your own custom spots using Google Maps and it gives you a forecast for that spot (averaged from the closest grid forecasts). They have 7-day forecasts but I have found that they only get accurate a few days in advance. And by accurate, I mean scarily so! The site gives 3-hour intervals for temp, rain, high/med/low cloud cover, wind speed & direction.
I think the main reason I find it so reliable is that it's much better at describing the forecast to you due to the wealth of information presented than a simple "partly cloudy 30% thunderstorms" type of forecast. It also takes spot altitude into account, so if you mark a spot on the escarpment, the forecasted temps will be about 10 degrees lower than the forecasts for Cathedral Peak Hotel, for example.
e.g. From these forecasts, you can tell that between 11am and 2pm they predict 3mm of rain, 0% high cloud cover, 0% medium cloud cover & 70% low cloud cover. In the berg, this means you're probably going to be above a sea of clouds in beautiful sunshine on the escarpment.
And no, I don't get commission!