Free Plugins!!

Native RTAS

That's right, completely free and usable within Pro Tools. Have at it!
Disclaimer: While all of these plugins are free, that does not make them good or useful. I, presonally, do not use many of them as I don't find them all useful for my particular situation. YMMV.
Plugins that I actually recommend are noted with a *

MAC Universal Binary =  
MAC OSX =  
Windows XP =  

Effects:

• Massey L2007 Mastering Limiter*
• Massey Tapehead*
• Massey VT3 EQ*
• Massey CT4 Compressor*
• Digidesign SignalTools*
• Digidesign Time Shift*
• Digidesign EQIII* (installed by default with newest versions of Pro Tools)
• Digidesign Dynamics III* (installed by default with newest versions of Pro Tools)
• Bomb Factory Free* (installed by default with newest versions of Pro Tools)
• Izotope Vinyl*
• Roger Nichols Inspector Free (formerly Elemental Audio Inspector... it's back!)*
• Nomad Factory Free (discontinued, not supported, only recommnnded for older versions of Pro Tools pre 7.x.. i.e. "you're on your own")

Instruments:

• Sampletank2Free (Don't forget to get all of the sounds/samples as well)
• Digidesign Xpand! (only available on CD from local retailers or for a nominal charge at the Digistore)*
• Synthmaster
• Remedy * A nifty and handy free synth.

Other

• Sonalksis FreeG (I haven't really found this useful yet, but... it's free!)

Pluggo: The following free plugins can be used as RTAS with the Cycling 74 Pluggo Runtime:

• VET-1 synth a handy basic synth.
• Phasetone
• Colortone
• Pluggo Jr. A free set of a dozen Pluggo plug-ins.

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VST

VST plugins cannot be used natively in Pro Tools, but with a small purchase ($99) you can get the FXPansion VST to RTAS adapter, which will "wrap" the VST plugins and allow you to use them within ProTools. Since VST is an open source plugin development, free vst plugins are abundant; some good, some very, very bad. (watch out for those). I have listed here some of my favorite free VSTs (all of these would have *):

• SSL Listen Mic Compressor*
• IKMultimedia Sonik Synth FREE*
• Anwidasoft Reverb Light
• PSP Audioware PianoVerb
• SIR Reverb
• Delay Lama* (Still haven't found a use for it, but so much fun!)
• Superwave P8* (eventually developed into the paid (but very reasonably priced) product, Superwave, but still free and cool)
• RGC Audio SFZ* free soundfont player; very decent for a free sampler!
• RGC Triangle I*
• RGC Triangle II *
• Camel Crusher *
• 
  Tons of others...