Tor (not TOR) uses directory servers to keep track of middle and exit nodes. You can get the list directly or via a variety of front-ends. See https://www.torproject.org/docs/faq-abuse.html.en#Bans

there's a DNSBL.

i believe it's possible to run a node and just query it for the exit nodes, but I'm not sure.

There's a nice updating list here: http://proxy.org/tor.shtml