The nice folk over at enature.com (National Wildlife Federation) have a great feature on their website: Zip Guides. Enter the zip code of the area you have been bird watching in and it will give a list of birds that are known to frequent that zip code. Click on the bird and you can learn and listen.
Excellent resource. Now if only I can find out who the bird is that I have heard but not seen for the last week in the backyard.