Random

Randomness in some UNIX operating system can be achieved through cryptographically achieved randomness such as arc4random(3) which uses the interfaces /dev/random, /dev/arandom and /dev/urandom. This method is assumed to be pretty good, more on this at RFC 4086. Weak randomness can often lead to the system compromisable.

Entropy sources

Sound cards and their white noise are a good means of entropy. Some CPU's provide random number generators which the Operating System may use. Crypto-accelerating cards usually provide a source of entropy.