RTAS (real time audio suite) plugins are plugins for protools systems. You can get free rtas plugins for protools if you know where to look. There are a lot of expensive plugins out there, and there are a lot of plugins that you get bundled with your protools systems.

here is my list of free rtas plugins for protools:

Avid plugins
a list of free avid plugins

native instruments:
http://www.native-instruments.com/#/en/products/?category=1339
guitar rig player – free version of guitar rig
kontact player – free version of kontact sampler
micro prism – free synth
reactor player – free synth

amplitube free
http://www.ikmultimedia.com/amplitubefree/download/

sampletank free
http://www.ikmultimedia.com/products/sampletank/index.php?pp=sampletank-2-xl-versions

Amplitube custom shop

bluecat bundle.
http://www.bluecataudio.com/Products/Bundle_FreewarePack/

flux stereo tool :D
http://www.fluxhome.com/products/Freewares/stereotool

amplion free

there are also loads available from

dontcrack.com
http://duc.avid.com/showthread.php?t=225019
http://www.protoolerblog.com/2010/01/02/the-free-rtas-plug-ins-list/
/some-free-legit-au-rtas-tdm-and-vst-plugins