Psyke83's on the Ubuntu Forums wrote a 'script' to do this for PulseAudio.

Ubuntu 10.10:

Currently the easiest method is to install from a deb created by WebUp8.

Ubuntu 10.04 and below:

There is a PPA containing the equalizer:

sudo add-apt-repository ppa:psyke83/ppa

sudo apt-get update && sudo apt-get install pulseaudio-equalizer

This list is roughly sorted from simple to professional and of course far from being complete.

Alsaequal

Alsaequal is an equalizer plugin for the (usually preinstalled) command-line audio mixer alsamixer as well as amixer.

Installation

sudo apt install libasound2-plugin-equal 

Start

alsamixer -D equal

amixer -D equal

To change to the equalizer in the running program, press F6, choose enter device name... and enter “equal”.

Configuration

You can find configuration tipps on wiki.archlinux.org, e. g. how to save and load custom presets.

PulseAudio Equalizer

Installation

Ubuntu 14.04 Trusty and 16.04 Xenial

Follow these instructions on webupd8.org.

Ubuntu 18.04 and later

sudo apt install pulseaudio-equalizer

Start

qpaeq

If you encounter an error and are told to make sure you have the pulseaudio dbus module loaded follow the instructions in this answer.

JackEQ

Installation

sudo apt install jackeq

Requirements

JackEQ needs a configured and running JACK Audio Connection Kit, a howto is provided by this article on libremusicproduction.com.

JAMin

JAMin is the JACK Audio Connection Kit (JACK) Audio Mastering interface (…) designed to perform professional audio mastering of stereo input streams. ^source

Installation

sudo apt install jamin

Requirements

JAMin needs a configured and running JACK, a howto is provided by this article on libremusicproduction.com.

JACK Rack with LADSPA effects

JACK Rack is a sound studio rack where you can store and combine LADSPA effect plugins. A collection of plugins is contained in the ubuntustudio-audio-plugins package. More on ladspa.org.

Installation

sudo apt install jack-rack

Requirements

JACK Rack needs a configured and running JACK, a howto is provided by this article on libremusicproduction.com.

Advanced audio software – normally using the JACK Audio Connection Kit – of course also comes with equalizing features. As I feel like this goes beyond the scope of this question, here's just a short list with links.

• 
  Kwave: sudo apt install kwave
  


• 
  MusE: sudo apt install muse
  


• 
  Audacity: sudo apt install audacity
  


• 
  LMMS: sudo apt install lmms
  


• 
  Rosegarden: sudo apt install rosegarden
  


• 
  Ardour: sudo apt install ardour, needs plugins for equalizing features


• 
  Qtractor: sudo apt install qtractor
  


• Renoise


• ocenaudio

Further information about audio software can be found on the German Ubuntu wiki wiki.ubuntuusers.de.

I was looking for a graphical eq package for Ubuntu 12.04 and found this post. Thanks a lot for this!

Unfortunately the link provided for WebUp8's deb is no longer valid, but there's this one that I found hosted on UbuntuUpdates.org and works perfectly.

I found that the easiest way of installing it is via the deb package. Installs right away w/o hassle. And this little thingy works, eh? Even stays permanently after a reboot! System-wide all the way**.

**Actually, I realized why: the eq application is just a frontend for adjusting the DSP's eq settings in PulseAudio; so mainly it's PulseAudio (ALSA?) retaining the settings - the eq interface is just there to allow accessibility to these settings. (I'm a total Ubuntu noob, hope this makes sense)

Too bad it's no longer supported, I wonder why not. But works on Precise alright.

I've recently described it here: Global_equalizer_for_ALSA

You can install PulseAudio with system-wide equalizer support. This is basically an update to the old PulseAudio System-Wide Equalizer. There are more details in this blog post.

If you want to enhance the sound quality itself you can take a look at https://r3dux.org/2013/12/how-to-enable-high-quality-audio-in-linux/.

For convenience I'll summarise it here.

Edit /etc/pulse/daemon.conf and look for the following three lines. They may not be in the same place and they may be commented out.

; resample-method = speex-float-1

; default-sample-format = s16le

; default-sample-rate = 44100

Uncomment and update them to the following

resample-method = src-sink-medium-quality

default-sample-format = s24le

default-sample-rate = 96000

Finally restart pulseaudio (and possibly your music player(s))

pulseaudio -k

pulseaudio --start

For the resample-method you can also try src-sink-best-quality but that uses around twice the CPU time on my machine, with little noticeable difference to the medium setting.

On my machine (Intel Core i5-3210M 2.5GHz 3MB cache) pulseaudio hits around 50% when using the src-sink-best-quality resampling method and around 20% on src-sink-medium-quality while playing FLAC audio.