Equalizer APO is an open-source audio equalizer for Windows.
The program is implemented as an Audio Processing Object. This allows it to integrate with Windows, and means it should automatically work with any application which uses the regular audio APIs (which is almost all of them).
Equalizer APO is a professional-grade tool with support for unlimited filters, and works on any number of channels. It's also carefully coded to have a very low latency, ideal for interactive applications.
What you don't get here is any beginner-friendly GUI for setup and configuration. To make the program usable to the not-so-geeky you should also install a front end, something to handle the tricky bits for you.
For example, if you install Equalizer APO, you can then install Peace Equalizer and it'll automatically detect and integrate with the system.
Open Peace Equalizer, choose the simple interface to start and you'll find a familiar equalizer interface. Choose a preset like Bass Boost, maybe drag the preamp slider to increase volume and it'll work immediately.
A professional quality and ultra-configurable equalizer. Keep in mind that it's not much use on its own, though-- you'll need a front end like Peace Equalizer to set it up properly.