Well the best all round tool is certainly baudline.
It's expressedly made for real time spectrum analysis ,noise
measurement of all kinds .. i mean it's worth quite a long look.
Trying it might get you hooked.
The best the sound card the more you'll get out of it.
http://baudline.com/
it's free :)
Ric
Baudline is a real-time signal analysis tool and an offline time-frequency browser. It
has a built in tone generation capability and it can play back audio files with a
multitude of effects and filters. Designed for environmental analysis missions that range
from modulation parameter measurements to searching for transient signals that go bump in
the night, baudline combines fast digital signal processing, versatile high speed
displays, and continuous capture tools for hunting down and studying elusive signal
characteristics.