Few years ago i noticed Sim Ant in a sales bin at one game store and it didn't have a price tag on it.
So i went to the counter and asked about it and the clerk gave it to me.
In the end the whole thing cost me 0.10EUR and that was because i took a bag for it

EDIT: typo corrected

Technically you still can get Sim Ant for free online if you know where to look :P
The best bargain I ever had was Space Rangers 2 Complete. For a mere USD4, it provided weeks of insane addiction

Getting the Orange Box for $25 a month after release at a Best Buy Thanksgiving weekend sale was probably the best deal I've ever gotten.

Orange Box period. Best deal.

I got Orange Box the week of release at Best Buy for $25.

Same here. I couldn't believe it when I saw it. It's one of my favorite games.

I got The Dig for £4 in HMV about six weeks ago. Quite the bargain back then, perhaps not so much now (although I prefer having the CD than having to deal with Steam).

Pure and simple: GOG.
Second to that:
City of Heroes Collectors Edition on clearance at Target for $10.
I added it to my account, got the CE bonuses AND a free month. :)
I do a lot of clearance shopping.. some of the games are dogs, but sometimes I get some winners. Got Titan Quest and it's expansion (so basically Titan Quest Gold), for under $20, shortly after TQ Gold released and was still like $30.
Also noteworthy:
I've not been able to confirm it, nor find anything to disprove it, but supposedly Microsoft has made the MechCommander series free.

Age of Empires Gold Edition. Cost me 6EUR (Euros). Damn, what a good game...

Morrowind and expansions: 4Eur (3,5$ aprox.)

STALKER for 5 USD.
Half-life 1 for 1 USD

Deus Ex big box edition in perfect shape for 6€.
Gothic 1+2 Collector's Edition for 10€. Two great games + bsos + goodies for the price of a meal.
Baldurs Gate Compilation for 12€ (this little box is like crack on a box)

Were those Steam deals? I think I got Half Life 1 for $.99 as well.

Thats too expensive. They have to pay me to take it. Worst. Game. Ever.

Syndicate Wars for R$16,00 many years ago (1997 to be exact). That would be like 8USD.