Fortunately, there's been many great software developers that have come out with 100% free RTAS-compatible plug-ins; all you need to do is download and install -- and you'll be on your way to making stellar mixes.
These plug-ins are all fantastic quality, and offer processing on par with many expensive plug-ins on the market. No, you won't get SSL-quality EQ or Waves-quality compression, but you can get close to top quality -- especially given the price!
Bookmark this page -- as we learn about, and test, more free plug-ins, they'll be added to this list!
Free RTAS Plug-ins
Click -- by JK Audio -- Click to Download -- Click by JK Audio is a free, Mac-only, RTAS click track plug-in. It's a variable metronome application that allows you to have a much more controllable click track within your Pro Tools sessions.
ColorTone Free -- by Tritone Digital -- Click to Download -- ColorTone is a fantastic free application -- it allows you to easily add analog "warmth" and "color" to your digital mixes. This gives your mixes an extra edge compared to a fully digital mix; you can add as much color to whatever you wish. While not as full featured as the "full edition" of ColorTone, it's pretty close. RTAS and VST compatible for both PC and Mac.
Free Bomb Factory Plug-ins Suite -- by Digidesign -- Click to Download -- One of the absolute best deals in free plugins is the free Bomb Factory plug-ins for Pro Tools. These are Mac and PC compatible, and offer some of the best free processing around! The Bomb Factory suite contains the Bomb Factory BF76 compressor (a true emulation of the vintage 1176 compressor), the BF Essentials Clip Remover, which will help repair clipped recordings; the BF Correlation Meter, for phase correlation; the BF meter bridge and noise meter, both metering applications to help better bridge the gap between digital quality and analog metering; and two other extremely useful plug-ins that I use in virtually every session I mix: the BF Tuner, a fantastic free instrument tuner, as well as the Funk Logic Mastererizer, a lo-fi effects app. These are some of the best plug-ins you can get at any price, especially free!
Stereo Tool -- by Flux -- Click to Download -- Flux Stereo Tool is a very unique plug-in. It gives you not only full visuals on your recording's stereo image, but it also lets you adjust the width and phase of the recording. Stereo Tool lets you manipulate stereo recordings (such as live two-track recordings) to sound bigger and better -- something you'll definitely want to play with in your mixes. I also find that Stereo Tool lets me manage tracks such as drum overheads and vocal submixes a lot easier.
Vinyl -- by Isotope -- Click to Download -- Vinyl by Isotope is one of the more impressive free plug-ins. It allows you to add analog vinyl saturation -- including pops and clicks -- which can either be applied to overall mix aesthetics or, in extreme use, for special effects within songs themselves.