I use Swellwatch.com - it's free and it provides 7 day graphical predictions of swell height, swell period and wind. I've found it to be very accurate 2-4 days out but occasionally the timing of a good/bad prediction will shift by half a day or so - if a future Sunday is predicted to be awesome on a Monday, it may turn out that it's actually Saturday that is good. However, their predictions are generally quite good and if there is a 2-3 day window of great conditions predicted, I can pretty much count on the middle of that prediction being great. Once it's within 48 hours, I find very few errors with the predictions with the exception of occasional winds being a bit higher or lower than predicted.