Look at http://www.peterwhitecycles.com for serious dynamo stuff.
The Shimano Nexus dynohub is a very good generator -- the page at Myra-Simon assesses the performance of the older NX-10; the current NX-30 is much better --, and the Schmidt is outstanding. If you want something cheap and don't want to build a wheel, get one of the Dynotec generators with a Lumotec headlight. All the above generators will offer years and years of reliable performance.
There are a few flaws with the generic $10 generator shown at the top of the page:
- much more resistance than needed, because of poor bearings, etc.;
- no built-in voltage limiter; you will have to pedal under 10-15 km/h otherwise you will have more than 6-7 V and you will fry lamps.;
- poor mechanical parts mean you will fry the generator soon.