A list of free effects for platform and software version would be welcome, but right now you'll have to do some digging. You could check Toolfarm's CS5 Compatible 64-Bit Plug-ins, part of the Toolarm Plug-in Finder, but you may not find free filters for CS5 there, like Video Copilot Reflect and Sure Target 2. AE Portal has an overview of AE filters labeled free by vendor and platform, but it's out of date.
Maybe the best bet is Mylenium's After Effects Plug-In Guide, which has an Alphabetical listing for Free. You can click on an entry to find what versions are available, though you're on your own finding the free filter through the vendor's main website. You may have to check the Discontinued list to see if it's been discontinued (like eLin). Mylenium also has a CS5 Compatibility listing, which can be missed if your browser page is not wide enough.
Even with Mylenium's help, you'd be missing some free "filters" if you skip the many free Pixel Bender, Custom Effects, or even Presets in After Effects.
By the way, if you haven't already noted them, check out FilmmakerIQ's 202 Adobe Premiere Pro Tutorials and 1001 Adobe After Effects Tutorials.
Update: see Free After Effects plug-ins for previous surveys of free filters before CS5. If you miss the Unmult filters, you might try Removing black or white from layer/footage, deMult/unMult at Motion Graphics Exchange.