I've been going through some keys that I got recently and I've found some new discoveries - new to me anyhow. Hopefully others have discovered these (or will in future trades or RRs).

Mandalay Bay tan key is cpi 2006594 11/02
Mandalay Bay blue key is innovative
Mirage Bare keys - the word "Bare" - one is red, one is black, and the backs are different

Esther