From my days as an amateur short film-maker, I frequently used a program called Audacity to create sound files for foley, voice-over, or any number of other applications involving live sound. For voice-overs Anthony has already created a really nifty utility which makes Audacity unnecessary for that application. However, the program makes for a really handy tool in creating a variety of sound files, as well as exporting them to a variety of formats. If you would like to export sounds files as .mp3, you need a specific encoder, of which I found LAME MP3 Encoder to be the best:
http://spaghetticode.org/lame/LameLib-Carbon-3.91.sit
And the free download site for Audacity:
http://audacity.sourceforge.net/download/mac
For a PC, follow the above links and the appropriate download should be easy enough to find.
The interface of the program is very basic – click the red circle to record, the yellow square to stop recording, and the green triangle to play back what you just recorded. You can also use a variety of audio editing tools which have a similar interface to iMovie or Final Cut.
Ryan