This is a very clean, high-quality, high-gain electret mic preamp. I (Stelios) came to it after having built a few different preamps, but none of them sounded as good as this one. It uses the TL071 op-amp. The circuit was found at the electro-tech forum.
1x electret mic: (Note: For most applications, a good, cheap omni mic is the Panasonic WM-61; it has a flat response but doesn't handle too high SPL and is somewhat noisy. Similar more high-end (but more expensive) mic capsules are the Primo EM172 (lower noise floor) and the Transsound TSB-140AO).
1x battery (9v)
1x battery connector
1x jack (1/4") out
1x connector for the mic
3.9k (Note: for 25x gain; smaller for higher gain)
47ohm (Note: This one is not crucial, but will keep the op-amp from oscillating if a shielded cable connects to the output - shielded cables have capacitance to ground).
Pot: 100k (log, i.e. audio) (Note: increase the value for higher gain)
film unpolarized (can also be ceramic)
Board (Prototype or printable)
These could be helpful if you're trying the circuit out...