I've used Sound Forge from Sonic Foundry to do this all the time
It's since been bought by Sony... and I'm banning Sony since the whole root-kit fiasco... so it's hard to recommend it, even though it's a great program.
audacity appears very similar
edit:
eh, kinda pricey at $70. I think I got a version for free bundled in with some hardware -- like a sound card, iirc.