In addition to the previously mentioned methods, you could use one of the "slot machine" ideas. For example, seed a PRNG with one of the previous methods and run it as fast as your machine can until you hit a key on the keyboard. Another class of methods uses exogenous noise sources. For example, if you have an audio input, feed it noise and use the LSBs of the audio stream. Just a microphone in your office/lab should provide sufficient randomness. A slightly better source is an FM radio receiver with muting off and tuned to no station. For the truly paranoid, bias a semiconductor PN junction at the Zener point, amplify and digitize the noise, put a statistical monitor on it to make sure that the bit stream produced has the appropriate randomness, make several of these and XOR them together.