Even if you're not trying to have a butterfly garden, chances are you've seen these two.
The Woolly Bear Caterpillar
And The Leopard Caterpillar
The Woolly Bear caterpillar has a red/brown stripe in the middle that varies from one to the other. Legend has it you can measure how long winter will be from this stripe. Personally I prefer groundhog and shadow test but it's fun to believe. According to "Caterpillars of Eastern North America" by David L. Wagner they're one of the few caterpillars that can eat virtually anything. But they are more commonly found eating Nettle, Grass, Dandelion and Lettuce. The adults measure 5 cm in length and overwinter in wooden structures before turning into probably one of the plainest and boring looking brown moths ever.
The Leopard Caterpillar is probably just as common now, and eats similar low growing non-woody plants. This one pictured below probably hatched out on the New Jersey state flower, The Common Violet, Viola sororia. I've had a patch of this plant growing here for years and it's somewhat invasive in full sun. But you really can't complain about a 6 inch ground cover that doesn't require mowing. The Leopard Caterpillar is a bit bigger, about 7.5 cm long. It over winters in wooden structures, and the resulting moth is actually kind of pretty and worth Googling.
And while we're on the topic I may as well mention that I've seen the Brown-Hooded Owlet feeding on some Asters while in Delaware. Species name is Cucullia convexipennis. They're a very long caterpillar and actually eat the flower petals to Asters and Goldenrods. The moth is another brown one that blends in with the crowd ... and trees, but they do have a splash of red on their back.
Also I'd like to thank Jessaboo for recommending Caterpillars of Eastern North America to me. And for identifying the Brown-Hooded Owlet. I recommend this book for anyone on the east coast who's interested in caterpillars. A word of warning though, it's 500 pages long and might be overpowering someone not familiar with IDing by Subfamily, Genus, Species. The book itself admits that some genera just can't be ID'd through pictures alone. But it's a fairly complete list otherwise, has wonderful pictures, and even names common host plants for each of them. The one place this book could only improve is providing a little more information each specimen. But they could only write so much on each page.