I was searching for a Free audio editing software and i found Audacity. Audacity is a free, open source software for recording and editing sounds. It is available for Mac OS X, Microsoft Windows, GNU/Linux, and other operating systems.
With Audacity you can import and export WAV, AIFF, AU, FLAC, MP3 files etc. You will need to install LAME encoder library to export sound as MP3.
Record audio via microphone. it can record streaming audio on windows too. Choose Windows WASAPI
Cut, copy, paste and Delete - you can use the normal shortcut keys like Ctrl+C, Ctrl+V, Ctrl+X, delete for these operations
There is a big list of effects which you can add to the sound or music including Echo, remove noise, reverd, amplify, etc.
unlimited Undo and Redo options
It have a legacy version which will run even on the Windows 98.
Portable edition of Audacity is also available.
Simple and easy to use user interface with visualization of the sound.
With plug-ins you can add more features.
You can download it from here Audacity. The latest version available is version 2.0.5 released on October 2013.