With martini in hand...I'll start. Eventide Ultra Channel. Got it...goofed with it...forgot about it. Then made myself go back to it and use it on a couple of projects just to learn it and derned if it didn't turn out to be a great piece of kit. Use it all the time now.

Your turn.

Camel Crusher. I've been using that one for over a decade now. It's one of my favorite saturation plug-ins. Probably my most used freebie, considering how long it's been around.
 
Eventide Stereo Room.
Lexicon MPX (won it on Facebook).
 
I haven't had the chance to use them much, but I was pleasantly surprised with Focusrite Red 2 and Red 3.
 

Duplicate.
 
You're drinking Martinis and I'm the one seeing double (posts).

Soundtoys Sie-Q
A Very Boutique EQ, mmmmmm, nice.

Ultra Channel, Camel Crusher, both amazing. 
 
But I think my favorite is Sie-Q. I did buy the Soundtoys bundle in a sale not long after I got it, so I would have gotten it anyway, but I was so pleased to get such a fantastic sounding EQ for free. It does amazing things to the bottom and top end and the drive control is amazing. In fact I started going through old projects and replacing other EQ's with it. Amazing for vocal air, kick thump and also mid-range on guitars. 
 
I have Eventide Stereo Room too, that one is great. 
 
I think my most often used freebie though is Tone Projects' Bass Lane. All it does is center frequencies below a threshold that you set. Sounds simple but I can't imagine what I'd do without it. It solidifies stereo sounds that have bottom end on them and is especially useful for things like stereo bass patches where you only want the mids and the highs to be stereo. It also works great on the master bus as long as you use sparingly and set the threshold low. Just puts all the bottom end right in the middle where it should be. The only thing I don't like is that it's 32-bit, but mind you, Sonar seems to play nicer with my 32-bit plugins these days for some reason. I've often looked for a free 64-bit equivalent but have always drawn a blank. There seems to be nothing else like it for free. 

Oh and I almost forgot, if you have the full version of Reaktor then Boscomac's free Reaktor ensembles are almost too good to be true. There is some absolutely top quality stuff in there - effects and instruments - that he could be charging good money for if he felt so inclined. It's actually donationware, so send him some money if you like them. I get so much use out of them that I donated, although theoretically you can use them for free so I hope that counts. 

.

Lately trying/liking the free Reaper compressor (free Reaplugs bundle, compressor, multiband, delay and some more stuff).
Had the 40 bucks Kotelnikov GE compressor but somehow never could get the gain right, really a shame because other than that it's great I think (they have a free version and a dynamic EQ too) <- Tokyo Dawn Labs.
 
 

SPAN
Kotelnikov
BlueCat Gain Suite
Martinique Combo-V (no longer available) *
Zebralette
Mini-Mogue *
OBxD
Kjaerhus Classic Chorus *
Kjaerhus Classic Compressor *
Voxengo Boogex
LoudMax
Pancake
 
* 32-bit only
 
Roth-AIR, the recent KVR DC entry, has gotten some use but it's too soon to tell if it'll make the permanent roster.

Cable Guys Pancake.