What entropy sources are used by the Linux kernel?

Most commodity PC hardware has a random number generator these days. VIA Semiconductor has put them in their processors for many years; the Linux kernel has the via-rng driver for that. I count 34 source modules in the drivers/char/hw_random/ directory in the latest source tree, including drivers for Intel and AMD hardware, and for systems that have a TPM device. You can run the rng daemon (rngd) to push random data to the kernel entropy pool.