One of the most common questions I get is “I saw an awesome butterfly–what was that?”
Here’s a great resource: Moths and Butterflies of North America
In addition to photos of most adults and larvae, you can even find what species are in your county! Nifty!
It also gives you info on larval host plants, for your garden planning.
You will need to know a little taxonomy to get the most out of it–things are organized by families–but you can also just pull up your county and look at all the pretty photos.
Have added this to the blogroll for your future reference. :)