Audacity
Is a FREE cross-platform digital audio editing program that lets you record, edit and mix narration and music. For both Pc and Mac users.
Goldwave
A relatively inexpensive digital audio editing program available for Windows-based PCs. It contains even more features than Audacity, that allow you to:
Adobe Audition 2.0
Adobe Audition is a very powerful and much more expensive professional grade audio editing application. It is recommended for digital audio professionals and serious amatuers. (Windows users only)