VideoPad Video Editor
One of my favorite features is the set of audio editing tools. VideoPad Video Editor includes multi-track sound-mixing software that lets you blend voice-over, music, sound effects, and pretty much anything else you can dream up.
Perhaps the coolest thing about this app is that NCH offers a free version for noncommercial use. If you want all the bells and whistles, the Master's Edition is available online for $69.95