First on the list is www.HAB-HAM.org
These guys are just starting up a site but have years of experience. They host a forum where you can get great tips on launching High Altitude Balloons. Be sure to look through and maybe become a member so you can talk with the other members.
This project was done by a young group of guys and they detailed the entire process from beginning to end. This should be a great help to any amateur balloonists looking for some how-to information.
HabHub.org - This is a great resource to help with pre-launch calculations and information. There are multiple tools available. This site was originally developed by the Cambridge University SpaceFlight team (CUSF).