Two more, which we use for all kinds of bugs, not just butterflies.http://www.whatsthatbug.com
This one is a lot of fun. Not very easy to use just for an ID, but you can submit a photo and hope they post it or reply. They've posted a couple of our photos, including a couple I took last year of a monarch caterpillar forming its chrysalis. http://www.bugguide.net
If you can't find an ID for the butterfly or other insect you have, you can submit a photo in the ID section and other users will respond.
There are also insect ID groups on flickr.