AmpliTube - Demo Download
Amplitude has a free download of their Tube Amp modeling software. This is a plugin that you can use with any Audio editing software that supports plugins. You can get Audacity multi-track audio editor for free from Sourceforge - http://sourceforge.net/projects/audacity. Audacity is a great package to edit your sound files and it's free. Now, with this module, you can model the amp with Audacity.
AmpliTube - Demo Download