Audacity is a freeware, open source, cross platform digital audio editor. Audacity is very popular is the podcasting world due to its wide availability for Mac OS X, Microsoft Windows, GNU/Linux, and other operating systems.

Audacity is used to record live audio through a microphone or mixer, import and export WAV, AIFF, AU and Ogg Vorbis files, convert tapes and records into digital recording or CDs, edit and add new effects with LADSPA plugins, and change the speed or pitch of a recording.

Major Audacity free digital audio editor features:

• Record live audio.
• Convert tapes and records into digital recordings or CDs.
• Edit Ogg Vorbis, MP3, WAV or AIFF sound files.
• Cut, copy, splice or mix sounds together.
• Change the speed or pitch of a recording.

More details about Audocity features

Download Latest Audacity – Free Digital Audio editor

PS: To add extra effects to Audacity, you can download and install plug-ins from http://audacity.sourceforge.net/download/plugins