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?
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.