There are a lot of coaching websites out there. In football, you have sites up for just about every age group and for every type of offense and defense imaginable.
For my money, the best one I've found by far is good ol' Coach Huey, which was thankfully one of the first sites I found when I made the decision to become a coach. Imagine a free online coaching clinic that lasts 24 hrs a day, 365 days a year. I strongly recommend that all coaches become a member and start participating.
Besides featuring posts from experienced football coaches from around the world who have experience at every level, from Pee Wee to the NFL, this moderated (thank goodness!) site has a sub forum for just about everything as well as a great articles section. It's a great site for anyone looking to learn about and discuss the game from a coach's perspective, and it's one of the best networking resources available in the field.
Yeah, it's that good. Check it out!