Anything hardware can do, software can do - if not better, then at least as well. Some of the most fundamental hardware tools found in a recording studio are compressors and limiters, so it makes perfect sense that there's a wide variety of software equivalents to choose from. The applications of compressors and limiters software are many, from boosting the sustain of a guitar chord to reining in the bass so it mixes better with the other instruments in the mix. For broadcast use, dynamic range compression can be necessary to make the audio fit within the range of the signal while still delivering the desired volume to the listener. And these examples are just the tip of the iceberg!
Most of the software packages in this section are plug-ins designed to work with one or more DAWs or some other flavor of mastering software. Cakewalk ProChannel Concrete Limiter, for instance, is built specifically for the Producer versions of SONAR X1 and X2. PSP VintageWarmer 2, on the other hand, works with ProTools as well as any application with VST support, or Mac programs with AudioUnit support - and its high-quality analog emulation makes it one of the best sellers in the whole section. You can also find some purpose-built compressors and limiters here, such as Melda Production MDrumLeveler Software for percussion.
It's all a matter of finding the right compressors and limiters software to suit your needs, and once you start narrowing down your options based on your preferred DAW and picking out the ones with the individual features you like, it won't be tough to track down the perfect fit. At the end of the day, choosing software is as personal and subjective as choosing hardware was for decades before it, so there's no choice that's objectively right or wrong - it's up to you, and as long as you're deciding carefully, you won't be disappointed.