
There are tens of thousands pages dedicated to C25K. I will give you some links that helped me the most with my C25K program and running related websites:
Cool Running: The Couch to 5K Running Plan. These are the guys that started it all.
Active.com: Section only for runners
Runner's World: Running, Racing, Training and more
Road Runner Sport: World largestrunning store
Runnersweb.com: The complete running and triathlon resource site
Runner's Gazette: America's first running newspaper
CARA: Chicago Area Runners Association
RRCA: Road Runners Club of America
IAAF: International Association of Athletics Federations