Some operating systems do provide a mechanism for generating random numbers from input which is, in practice, impossible to reproduce or guess. This is done by collecting data from hardware events such as network device and disk interrupt latencies and then generating numbers based on the contents of this pool.

/dev/random and friends are useful for seeding PRNGs, not necessarily useful for all your random number needs (they tend to run out rather quickly: try generating a key of some size with GPG, and spending half an hour tapping the ctrl key to give it sufficient randomness). If you want reasonably strong (unguessable) randomness, without waiting for the OS entropy pool to fill up, you'll need to consider which PRNG you're using.

And for that, I refer you to chapter 16 of Bruce Schneier's Applied Cryptography.

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