NOAA's Office of Coastal Survey (charts & updates, coast pilots, etc) - 

NOAA's National Marine Weather Service -

Seven Seas Cruising Association -

Landfall Navigation (awesome site for gear) -

Bluewater Books & Charts (great site for, you guessed it, books & charts) -

Discount Marine Electronic Wholesalers -

Doug Ritter's Equipped to Survive Organization (Safety) -

Sea School (Training courses) -

United States Power Squadron (regional & local clubs) -

Boat Trader Online (find your perfect boat) -