The bx_limiter from Brainworx is a simple but effective low latency limiter plug-in with features that make it an excellent mastering tool, but also useful for mixing duties. Use the dry/wet control to smash up drum beats, pull up vocal details in mixes, control bass sounds, etc., and then seamlessly blend from linked to unlinked mode for stereo mastering.
Additional processing can be achieved by driving individual tracks or stems hard into the limiter to trigger saturation while still maintaining overall levels. Parallel processing can create vintage drum and vocal sounds or special effects of all kinds. And in this case it is important to keep multiple-track peak levels under control, rather than trying to do all the work on the master buss.