Dynamics Processors

Compressors, limiters, noise gates, companding and other dynamics processing are still handled in the analogue domain for performance advantages, before the signal is converted into the digital domain. Analog Engines from THAT Corporation are an ideal building block and THAT provide many supporting application notes to steer you through the possibilities.
The ideal op-amps to accompany the Analog Engines are either the NJM8068 for ±15V voltage rails, or the NJM8202 for +5V rails or similar.
In the digital domain, DREAM offers an extensive free-to-use library of dynamics algorithms for use with their SAM5000 DSP range.