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.
a list of free avid plugins
guitar rig player – free version of guitar rig
kontact player – free version of kontact sampler
micro prism – free synth
reactor player – free synth
flux stereo tool :D
there are also loads available from