Way too many to name but the best was the Yellow control deck test cart I raffled off a few weeks ago. Got it for $6 at a flea market.

Other highlights-- Bubble Bobble 2 under $5 at a rental store,-- Dragon Warrior IV and a CIB Blue box Action set for $33.--
A CIB black box action set $25,
CIB-- L'Emperuer, Conflict, Gengis Kahn $5 each at Goodwill--

CIB's of each NES-- Final Fantasy, Tetris 2, Dr. Mario, Ultima 3,Werewolf (SNES)-- Final Fantasy 3, Secret of Mana, Breath of Fire, Megaman X2, Donkey Kong Country 1-3, SNES System, Shadowrun, Zoop, (about 4 other games I don't remember) (Genesis) Sonic 1-3, X-men,Phantasy Star IV, Shadowrun. And several other games I don't remember that well.. All for $125!

that last one was the deal that really kick started my collecting habit because I made about $500 on the stuff I didn't want to keep (Ie the non NES games)and realized I could make a lot of money from buying stuf and reselling it which means I could afford to buy more NES games. after that I spent almost every weekend at the flea markets.

12 NES games 30 $ and I got a free M82 included

Oh yeah, count me in on a Bubble Bobble 2 cheap as well. I still have the price sticker from 2 Christmases ago. $6.95

I'd like to think my best finds ever haven't happened yet, since I pretty much spend my idle Saturday time out in the "wild" hunting. To date though, scoring a Maxi-15 in the hard case for $10 probably tops the list. $5 CIB Final Fantasy (still in the wrap to boot, everything is as mint as anything I own), $3 G.I.Joe, $2 for Mega Man 6 and $4 Mega Man 5s (two of them for this price in fact).

I bought a CIB Uninvited for $7 shipped on gametz.com some years back, still think that was a great deal too.

Got a DS for $10, and that was 2 years ago.

Miracle Piano and Miracle Piano game for free.

my old roomates boss heard that I collected nes games so he gave me peekaboo poker for free. That was years ago and had to sell it for rent. Sad sad day!

For $25 (that started it all)

All for $75  Included a mint Mario paint CIB, Mint SNES CIB and 1-80 NP issues

my how i've had many finds over the years
One significant haul that got my off and collecting was a video store that went under. They had EVERYTHING up through PS1, but i was a poor kid just outta high school. I ended up snagging 300+ CIB NES games for $211.00... i still have the original check back i wrote out for that one somewhere...
After that, bought out three more rental stores, one for $300 and the other for $1k. At that point i almost had a complete CIB NES NTSC set, including all of the known (at the time) color dream variations complete.

Some of the stellar scores in the mix:
FFF Athletic world -$10 shipped - ebay - before anyone knew about it , sold it to Dain as he was a smart smart man when i was selling out.
SE Manual and FFF Pad - $265ish or so (can't recall) - ebay
Cheetahman II - $125 ebay
Caltron box/manual/sleeve - $10
Myriad box/manual - $10

and that's just NES wise. I have found some of the hardest to find A800 and C64 game cartridges/disks (complete ones too) in the wild and for next to nothing. I've made some good good money on video games over the years and will always be grateful for this hobby.

Now i pretty much pay market value on stuff as I don't have the hunt in me as I used to... but i still always love a good deal

^Myriad box/manual for $10....holy hell...

And I'm still grateful for that purchase Josh. I think you sold it to me for $120 (ish), which would (now) qualify this post at one of my best finds ever! You totally ripped me off dude! (he he!)

My best find ever was a Rodland cart for 10 Euros close second being a factory sealed F-1 Pole Position (Gameboy) for $75.

My best finds would be some of the service centers I bought out for cheapo or all the Wisdom Tree stuff I lucked into. I pretty much got Happy Camper proto for free
~~NGD

Honestly Dain, I knew it was rare, but i think you and I were the only ones who did at the time.  I had it posted online in my sale thread for well over three months with a "shoot me an offer" next to it.  I had no idea on a price for it and when you sent me an offer of 110 ish (from what i remember, i will have to dig through my old PMs over at nesworld) i was super excited, considering you were the only one to shot me an offer and one "so high".

btw, before i bought it, i had only assumed it exisited since there was a five screw cart and i the extra manual for it i had laying arond with the "series 1" printed on it (that i had gotten with the SE manual).  I searched for a copy of it on ebay day and night and when one popped up, i snagged it for nada cuz hey, i was only the one looking at the time

with sites like NA and everyone being aware of the extreme rarity of odd stuff, it is now near impossible to get deals like that anymore, but oh well.  I am glad it is well known and documented and the value on it is through the roof. I am searching for a sealed one if anyone has one for sale

Jason, The find of Happy Camper is one of the most important finds in the history of the NES.  Seriously. It was considered not to even exist.  Between Happy Camper and all your original Color Dream Artwork, i would say you have some of the best finds ever in this hobby.  Can't wait to see that Free Fall release and maybe James (Zzap) will start programming that Code Blue and we will have another CD release

Well, I've never had an amazing find like unreleased protos or high-end rarities like Cheetahmen 2, Caltron, etc, but I'm happy that I managed to find a great deal of mid to mid-high range rarities for (mostly) cheap.

-Bought a CIB FFF-AW in 2005 and a second one in 2006, both were something like $15 shipped.
-Got some crazy good deals from Captain Nintendo on RFG and DP years ago when he was more active. Double Strike, Ultimate League Soccer, Panic Restaurant, and Blackjack, all $5-10 each. Bought about 10-12 boxes from him a couple of times. all for cheap.
- MM1 box/manual for $1 each on GameFAQs board, along with some others. They were both in ~7-8/10 condition.
- Action 52 CIB w/o the comic in a trade for four fairly common games plus $20.
- Tagin' Dragon, Castle of Deceit, Mission Cobra CIB for $20 each on Ebay. It used to sell for $20-30 regularly on Ebay at the time. I bought a few of them and would trade them off for games that I needed. Bought my Mission Cobra from Qix in his big sell-off for $35 and it was in great condition.
-Got a load of rarer boxes from a guy on DP for cheap, RR2, Uncharted Waters, Bandit Kings, Gun-Nac and others.
-The entire DW set CIB along with at least 20 others in a thrift shop for $3 each, most of them were in great condition, DW4 was mint/near-mint easily.

Unrelated to NES:
-Vectrex and 10 games for $100 shipped.
- I used to sell VHS tapes on Ebay quite a bit. I bought a few in old rental stores for $2-5 and sold them for $75-125 in the past. Some of them are on DVD now so they don't sell for nearly as much. 
-Bought out an old store for their sealed / boxed SNES, NES, SMS, Atari, Coleco games years ago. They had some nos stock Xonox games still in their original packing boxes, three of them and one that was half full still.

FFVII Misprint for 2 bucks,
MvC 2 for dreamcast for 5 bucks
MINTY clear gameboy with the clear carrying case and all paperwork for about 3 or 4 dollars. That GB even had the cardboard advertisement in it.

Paid $260 for 21 factory sealed copies of Castlevania II: Belmont's Revenge for Game Boy. about 2 or 3 years ago. I only have 3 left now. And made more than 10 times my money back!