Jambase.com has a huge listing of all the festivals and good jam events in the US - and probably a good number overseas as well.
Langerado is in FL the first week of March. The lineup is awesome - www.langerado.com
Bonnaroo is in TN in June. www.bonnaroo.com
All Point West in North NJ...not sure of the website off the top of my head. Think its in late August.
Wakarusa in KS in the beginning of June. www.wakarusa.com
There's other smaller ones all around, use Jambase to search.