Multiples of America: Also known as the National Organization of Mothers of Twins Clubs. Find information and your local chapter! Also for parents of higher order multiples. You'll probably find that each state also has it's own organization and website, and the same for each local chapter within the state. Also check out various chapters Facebook pages!
City Moms Blog: Check to see if your city has its own blog! Not just for moms, these sites have city-specific reviews, events, and articles for parents. I've used the one in our city to find out about family-friendly activities and sights, and for general advice.
The Imagination Tree: creative ideas for fun and learning at home.
Hubbard's Cupboard: playdough recipes and sensory tub ideas for every month of the year.
Wholesome Baby Food: recipes, tips, menus.
BabyCenter: a TON of resources and advice on everything you could think of.
American Academy of Pediatrics: self explanatory :)
FlyLady: help on organization, housekeeping, time management, and decluttering
Meetup.com: Find groups with like interests in your area.
Twiniversity: parenting classes for parents of multiples, Multiplicity magazine, and other resources