This list is roughly sorted from simple to professional and of course far from being complete.
Alsaequal is an equalizer plugin for the (usually preinstalled) command-line audio mixer
alsamixer as well as
sudo apt install libasound2-plugin-equal
alsamixer -D equal
amixer -D equal
To change to the equalizer in the running program, press F6, choose
enter device name... and enter “equal”.
You can find configuration tipps on wiki.archlinux.org, e. g. how to save and load custom presets.
Ubuntu 14.04 Trusty and 16.04 Xenial
Follow these instructions on webupd8.org.
Ubuntu 18.04 and later
sudo apt install pulseaudio-equalizer
If you encounter an error and are told to
make sure you have the pulseaudio dbus module loaded follow the instructions in this answer.
sudo apt install jackeq
JackEQ needs a configured and running JACK Audio Connection Kit, a howto is provided by this article on libremusicproduction.com.
JAMin is the JACK Audio Connection Kit (JACK) Audio Mastering interface (…) designed to perform professional audio mastering of stereo input streams. source
sudo apt install jamin
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.
sudo apt install jack-rack
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.
Further information about audio software can be found on the German Ubuntu wiki wiki.ubuntuusers.de.