JAMin is a GPL-licensed professional audio mastering suite.

JAMin is the JACK Audio Connection Kit (JACK) Audio Mastering interface. JAMin is an open source application designed to perform professional audio mastering of stereo input streams. It uses LADSPA for digital signal processing (DSP). JAMin is licensed under the GPL.
JAMin features:
    Linear filters
    JACK I/O
    30 band graphic EQ
    1023 band hand drawn EQ with parametric controls
    Spectrum analyser
    3 band peak compressor
    Lookahead brickwall limiter
    Multiband stereo processing
    Presets and scenes
    Loudness maximiser
Here are some screenshots of JAMin in action:

Software requirements.
JAMin requires following c.omponents to be installed in your system:
    ALSA — Advanced Linux Sound Architecture
    JACK — JACK Audio Connection Kit
    LADSPA — Linux Audio Developer's Simple Plug-in API
    SWH plug-ins — Steve Harris' LADSPA plug-ins
    FFTW — Fastest Fourier Transform in the West
    liblo — an implementation of OSC protocol
    libsndfile — Erik de Castro Lopo's sound file I/O
    GTK — The GIMP Toolkit

Hardware requirements.
JAMin uses FFT, so it's a quite CPU and memory hungry beast. It is recommended to use the most recent hardware, 2 CPU if possible. However, it is reported that running an AMD Athlon 1700+ (1.46 GHz) with 512 Mbyte PC133 memory make it real running Ardour->JAMin->Ardour with 12-16 tracks plus effects with no problem.
Download.
JAMin is available as a tarball from http://sourceforge.net/projects/jamin. JAMin is a rapidly changing beast. Because of this you may want to get the JAMin software from CVS. If you decide to use CVS here are the commands to download JAMin:
cvs -d:pserver:anonymous@cvs.sourceforge.net:/cvsroot/jamin login
cvs -z3 -d:pserver:anonymous@cvs.sourceforge.net:/cvsroot/jamin co jamin
When you are prompted for a password simply press Enter.

Building and installing JAMin.
After downloading the tarball:

tar -xvzf jamin-*.tar.gz
cd jamin-*
./configure
make
make install

If you downloaded from CVS do the following:

cd jamin
sh autogen.sh
./configure
make
make install

Notice that we didn't specify --prefix=/usr on the ./configure line. This is because JAMin is a program and it only needs to be in your path to run. You can put it in /usr if you want. Just remember to be consistent — do it the same way every time.
Configuring JAMin.
JAMin has a single configuration file — jamin_ui. This file can be used to define GTK user interface customization. It is not required. There is an example loaded into ${prefix}/share/jamin/examples/jamin_ui when you do the make install. To customize that file, copy it to ~/.jamin/jamin_ui and make changes.
Installation.
If they are not present, add them (there should be no "#" character in the beginning of both strings) and run

sudo apt-get update

This will help apt to know, what new packages are available via updates and contrib repositories. Now run

sudo apt-get install jamin

Screenshots.