Best game on steam sale for about 3$

Ignite, Fortix

Jericho

Home, Defy Gravity Extended, Capsized

Terraria, maybe?

I'm going to suggest Zeno Clash, I enjoy it quite a bit. Better with a 360 gamepad though, even if you need to find config files to enable it.

Gish, Dark void zero, Half life opposing force, max payne

Witcher 1
Fable TLC

Ignite, Zeno Clash, Hydrophobia, Dreamkiller, Witcher

Freedom Force Pack :D

Terraria, it's pretty easy to get a hold of too

Dungeons of Dredmor Complete Pack if your friend is up for some challenge. Or maybe Critter Crunch for some casual time.

You can buy "Deux Ex: Human Revolution", is a very good game and now is just 3 euros (i don't know if is the same price in dolars)

Deus Ex: Human Revolution is €3 in here, could be $3 for other regions.

How about Bad Rats?

I'm just kidding, I wouldn't wish Bad Rats on my worst enemy.

Thomas was alone is an incredible game but I think that would cost you one dollar extra.

total war games

Awesomenauts when it hits 3.39

limbo

Binding of Isaac

Awesomenauts is 3 bucks right now

Super Hexagon!!!
Super addictive imo

Hotline Miami is $2.5 on Amazon now if you have the discount coupon.

Deus Ex: Human Revolution

This or this or this or this or this. Probably missing many other great ones, this is what I can provide on short notice.

Awesomenauts or Super Hexagon...
AWESOMENAUTS OR SUPER HEXAGON. YOU HAVE NO OTHER CHOICES!

I really like Awesomenauts, but it lacks a singleplayer, which I'm sad about :C