An Exiter adds harmonic distortion to the audio signal. Use it to e.g. bring the lead vocals right to the front of the mix when used subtle.
Neutron from iZotope is a mixing channel strip with EQ, Compressor, Exciter, Transient Shaper and Limiter. The presets and Track Assistant quickly gives you a good starting point and the Masking Meter helps to avoid frequency collisions.