the best RTAS freebies as far as I am concerned are:-
Inspector. I can't live without this plug-in. Ideal alongside Maxim for doing quick 'masters' of your output.
Plugsound FREE brilliant for adding a few (very good) sample patches to your computer. Far more stable than Sampletank FREE (another free RTAS) in my experience.
I find the 1176 and the phase correlation plug included with the Free Bomb Factory Plug-ins to be very useful. I also have call to use the clip remover from time to time but have never got it to work. But everyone knows about these don't they?
Frohmage is fun. I find it sounds better than Prosoniq North Pole which is also free for RTAS.
There are loads and loads of free Pluggos available (see Simon's DUC thread). I have found that loading your machine up with every single one causes serious stability issues so if I had to reccomend just one it would, without doubt, be Gleetchplug. Just brilliant brilliant and very usable & stable plug-ins (including an automatable theremin!). I would like to use the stereo spread plug on my master fader but have found that is not transparent enough. Hopefully Todor is working on it though.
Also look for a free software reverb called 'ambience'. This is a really good reverb.
ALSO: There is a free Impulse reverb plugin called SIR.
Search for it in Yahoo!
It has to be used with something that has plugin delay compensation though.