Audacity is a free and open-source, cross-platform audio editor and recorder available for Windows, OS X and Linux. Beyond the typical equalization options expected in a basic audio editor, Audacity includes a high quality noise removal filter that allows the user to select a brief moment of silence in a recording and use it to create a noise profile. When applied to the entire audio file, Audacity uses the information taken from the noise profile to remove hiss throughout the recording.
Though intended for use as a multitrack DAW, or digital audio workstation, Reaper comes standard with ReaPlugs VST FX Suite — a growing collection of VST plugins designed for Reaper — that makes it appropriate even for single-track audio processing tasks. To remove hiss and noise, try ReaEQ, a ReaPlugs VST effect with an unlimited-band graphical equalizer and a variety of configurable filters, including low shelf, high shelf, band pass, low pass, high pass and notch. OS X, Windows and Linux users can try an uncrippled, shareware version of Reaper before buying it.
For Windows users, both flavors of Sound Forge — Audio Studio and Pro — offer a variety of ways to remove hiss from recordings. Users can choose from paragraphic, parametric and multiband equalizers. The paragraphic EQ in particular contains a variety of presets aimed specifically at hiss removal.
OS X users with a serious interest in audio processing and mastering can remove hiss with Bias Peak Studio, a recording industry-standard suite of software programs designed with the professional in mind. Remove noise and hiss with Peak Pro, audio processing and mastering software with myriad filtering, processing and equalization options, or with SoundSaver, a utility aimed at restoring imperfections in digitized cassette and LP recordings.