I thought it would be handy to build a list of your favorite (ie: you actually use them) freeware Photoshop-compatible filters.

Criteria:
1) totally free
2) not demo version or "lite" version of pay software
3) Must have Mac and PC versions
4) Provide a link to the download page

I guess I should get things started:

Wacom Pentools (http://www.wacom-europe.com/uk/downloads/tools/tools.asp) (requires a Wacom pad, and I haven't tried them since I upgraded to Photoshop CS)

Eliminate White (http://www.edesign.com/filters/) (I only used this once, but it was the only solution at the time and worked wonderfully)

Polaroid Dust & Scratch Removal Utility (http://www.polaroid.com/service/software/poladsr/poladsr.html) (much better than the PS builtin filter, I use this one a lot)

It's the "must have Mac and PC versions" that's the killer... I think most of the truly free filters have been developed originally for personal use, and few of the developers run both OSs.

For PC and Mac

Flaming Pear Freebies
http://www.flamingpear.com/goodies.html
Scroll down to "Free Filters"

Cheers