Version 2.1.0 of Audacity, the free audio editor and recorder, was just released. Here are some of the improvements:

Real-Time preview for LADSPA, VST and Audio Unit effects

Lots of improvements in effects, including Noise Reduction, that now replaces Noise Removal

Redesigned meter toolbars

Spectral selection in Spectrogram view

Visit their sourceforge page for the announcement and for downloads.