OFFICIAL PAGE: Audacity
Audacity is a free, open source, multi-track audio editor and recorder for macOS, Windows, GNU/Linux and other operating systems. Audacity now includes some spectral editing capabilities. While Audacity doesn't include some of the more advanced spectral editing tools included with more sophisticated spectral editors, it does allow you to boost or attenuate selected regions and the spectral edit multi tool can apply a notch, high pass, low pass, or parametric filter to the selected regions.
Screenshot courtesy of Christopher Kissel