Well you're looking for a compressor/limiter then, depends how much you want to spend. Are you talking about one single mic, or a stereo pair, or a bunch of them?
There's the
Presonus Eureka channel strip / compressor at around $700 Or for $1200 you can get
8 channels worth if you want to stick with the same manufacturer.
FMR Really Nice Compressor (RNC) is $200
DBX is probably the best known name in comp limiters, they have a variety of models.
All of the usual suspects in the project studio biz have entry level products - ART, Samson, Focusrite, Presonus, the German copycat company who shall remain nameless here, Zoom, BBE, Alesis, etc. etc.
Myself I have a Mindprint
Envoice Mk. II channel strip, and a DBX 1066, plus some cheap Alesis units which I wouldn't use on voice.
You can easily spend thousands, however, on higher quality kit: Empirical Labs, Avalon, UREI, Universal Audio, SSL etc. Others here will have experience on these and can give more input