Love the concept behind this thread. In keeping with my tendency to look gift horses in the mouth, could I offer a suggestion?...
That only plugins that have been confidently voted (by at least one person) as being significantly better than other widespread alternatives be allowed into the top post. IOW, I love to hear suggestions of cool, weird, or new plugins, but I really, REALLY like SteveStrummerUK's original premise of listing the plugins he is keeping as he throws out all the other interesting ones he's accumulated. There is a very long thread of free plugins on this site, as on most recording forums, but they all grow extremely long and sprawling and full of "try it and see what you think" endorsements. I think there is value in keeping a short list.
In that spirit, my own suggestion for absolute best-of-breed freeware is the Simulanalog suite of guitar effects. Not much mastering application, but they are second-to-none when it comes to guitar effects emulators. No GUI, unfortunately, though! Download here:
http://www.simulanalog.org/ Cheers.