There's a list of 15000 most commonly used English words at
Sustainable English
It's based on a project from England though, so some British->American conversion will be needed before it's useful for most of us. Also need to add contractions and I'm not sure if it included plurals or not. Might be a good starting point though. I'm at work right now so I can't do more than post quick replies, but I'll be looking into this more later tonight.