OpenBSD Implementation
Strong Random Number Subsystem

Gathers entropy from IO interrupt timing
Mouse movements
Keystrokes
Disk activity
Audio playback and recording
Network activity*
Timing of the random number subsystem
And of course... hardware sources of randomness
Most newer systems have some sort of hardware randomness device

Entropy mixed into a pool
Estimate of total randomness in the pool is maintained

* Busier systems on the network generate more randomness