Do Pro Tools 8 HD plug-ins work on Pro Tools 8 LE? What Plug-ins will work on Pro Tools 8 LE that you don't get with your Pro Tools LE package that are freeware and acceptable with Mac OSX?
Here are some suggestions of free plug-in that play nice with Pro Tools on a Mac...
FreeG from Sonalkiss http://www.sonalksis.com/freeg.htm
Mid-Side Suite from reFUSE Software http://www.refusesoftware.com/products/feature/10
Flipper from reFUSE Software http://www.refusesoftware.com/products/feature/12
Vinyl from iZotope http://www.izotope.com/products/audio/vinyl/
BitterSweet 2 from Flux http://www.fluxhome.com/products/Freewares/bittersweet2
StereoTools from Flux http://www.fluxhome.com/products/Freewares/stereotool
bx_solo from Brainworx http://www.brainworx-music.de/en/plugins/bx_solo
bx_cleansweep v2 from Brainworx http://www.brainworx-music.de/en/plugins/bx_cleansweep_v2
Hope these help,
After a little more research I found this page from MusicFormation.com http://musformation.com/free-rtas-plugins.html
There is the SoundHack bundle as well http://www.soundhack.com/freeware.php
Audio Geek Zine have a list of their top 10 which includes most of the above http://www.audiogeekzine.com/2010/03/top-10-free-pro-tools-rtas-plugins/
and I wrote about some of them in Sound on Sound back in 2008 http://www.soundonsound.com/sos/feb08/articles/ptfeature_0208.htm
Great links Mike.
Also, to answer the first part of the question, all RTAS plugins are compatible with BOTH PTLE and PTHD.
TDM and TDM-HD plugins are compatible only with PTHD.
Try Massey www.masseyplugins.com. They have a free unlimited demo period with no beeps. The only problem with the demo is that it does not remember your settings (i.e. next time you start session your settings on the plugin will revert back to it's defaults).
I can't wait to dig into some of the items on Mike's list above.