Audacity
is free and the source code is available
under the GNU General Public License.
Audacity
is cross-platform - it runs on Windows,
MacOS, and many Unix platforms, including Linux.
No limits on the number of tracks or the length of any track,
except the size of your hard disk.
Import almost anything: WAV, AIFF, Next/AU, IRCAM, MP3, and
MIDI files are supported natively, but
Audacity
will also open just about any uncompressed sound file and
automatically deduce the format (using the
Import Raw Data...
feature).
Audacity
not only includes many high-quality effects built-in, but also
lets you use plug-in effects in the industry-standard VST format.
There are dozens of free, shareware, and commercial VST
plug-ins online that do everything from Reverb to
Noise Reduction.
Audacity
acts like a non-destructive editor, providing
multiple levels of undo, but it also writes changes made to the
audio to disk, eliminating the need for complicated real-time
processing.
Label tracks allow you to annotate waveforms (for example,
transcribing speech) and later export the waveforms to a text file.
Powerful spectral features allow you to view waveforms as
spectrograms or plot the power spectrum of any region of audio,
and even export this data to a spreadsheet.