I have a lay over in Frankfurt (FRA) Airport for eight hours, and my final destation is Lisbon, Portugal. I hold a US Passport. Will I be able to go outside of FRA Aiport and join a sightseeing tour?
If you're going to Lisbon, Frankfurt is where you're going to go through the Schengen border controls anyway. The flight from Frankfurt to Lisbon is within Schengen and doesn't involve passport checks (except what the airline does at the gate to make sure that the person who flies is the same guy they sold a ticket to).
Once you have arrived at Frankfurt and cleared the border checks you're free to leave the airport.