The New England Dragway.
Actually, I'd like to try the Sarthe (closed off), the Nuburgring, pretty much things that have been mentioned. I've been to Lime Rock, and it seems like a fun course that I'd try. For a famous course, it's in my backyard, too, relatively speaking... (2 hours)
I'd love to try the Cannonball Run, though. But that isn't quite a racetrack.
I'd also like to drive Route 6 from Cape Cod to California. I've heard it's an interesting drive. To do it in the 944 would be an amazing feat, and a lot of fun too.