Den Leader Support
Den Meeting Plans - BSA's year long schedule of monthly den meeting plans from the "Den and Pack Meeting Resource Guide"
US Scouting Service Project (USSSP) - "a scouting portal website created by scouters for scouters" This is a big, searchable site with information and ideas about everything from advancement lists to camping recipes.
Baloo's Bugle - a direct link to the USSSP's monthly collection of ideas for Cub Scout leaders
Cub Scout Academics and Sports Program - descriptions and worksheets for Belt Loops and Pins
Scouting Magazine - BSA's online magazine for adult volunteers and professionals
Official BSA forms
spread sheets - keep track of your den's advancements, belt loops, attendance, etc.
Meritbadge.org - has information for leaders and parents on everything related to scouting