I'd be lost without the Digital Atlas of the Virginia Flora. This online resource lists all the plants which naturally occur in Virginia, along with maps showing which counties they grow in. You can even get list which show all the plants for a particular county! I'm constantly going to this site to help identify the plants I come across on my plant walks. If you're into native plants, this site is for you.
This online atlas is part of a project to produce a modern flora specifically for Virginia (the last one was published in the mid 1700's) and will serve as a companion to the written manual. The manual is scheduled to be completed sometime around entire 2012.
Another great online reference is Alan S. Weakley's flora of the Carolinas, Virginia, Georgia and Surrounding Areas. This site has lots of great plant keys. I've printed out several keys to carry around and help me identify some of the more confusing taxa such as Solidago (golden rod).