There aren't very many electronic acoustical devices. Equalizers should be used sparingly and only when they do more good than harm. Usually they are used for correcting low frequencies and are parametric. Here a a few examples.
QSC DSP-4 - A very good compact stereo inexpensive digital equalizer. They make several other parametric equalizers.
Audyssey Sound Equalizer - This is Audyssey's stand alone 8 channel auto-setup digital equalizer. Audyssey is now built into many surround sound receivers and processors. It requires the manufacturers hardware and software to set up.
Rane RPM-2M - Another very good stereo digital equalizer. They make several more parametric equalizers.
