there are also different noise sources, generally external and internal noise sources. seems like everybody here has been thinking in terms of external noise sources. external noise sources are the easiest ones to deal with, using shielding, filtering, etc... there are also internal noise sources, and these are more difficult to deal with. resistors and active devices (transistors, etc) all generate a certain amount of internal noise. in fact, a zener diode (or a B-E junction in a transistor biased into it's zener conduction region) makes a very good noise source (if you want noise on purpose).
resistors create noise proportional to their resistance. transistors, FETs and vacuum tubes generate noise when in conduction. dealing with these noise sources and minimizing their effects is almost an art in itself.
some useful search terms would be:
shot noise
flicker noise
johnson noise
1/f noise
avalanche noise