[jbri]

In theory? Quantum and thermal effects inside an unstable configuration of transistors.

The simple example is a basic SR latch (two NOR gates, where the output of one gate feeds one of the inputs of the other, and vice versa), where you start things off by applying a signal to both S and R. When you remove the signals, the latch will eventually fall back into one of the two stable states - but which state it ends up in is random.

So you can easily produce a stream of bits from a potentially-biased random source, and then do some deterministic massaging of that stream to produced an unbiased stream of random bits.