This paper by David A. Wheeler is quite interesting:
http://www.dwheeler.com/trusting-trust/
It talks a method to counter a possible malicious trojan-horse to be planted in a compiler, which was presented by Ken Thompson in his ACM Turing Award acceptance speech. This trojan-horse is very special that by examining the source code you find no clue of any malicious code.
The idea is very cute!
