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Fallout New Vegas + Fallout 3

You could go with Deus Ex: Human Revolution. It's a pretty long somewhat stealthy game.

I was going to suggest the Darksiders, Skyrim, Borderlands and the Fallouts too, but you got all of them. Though you might want to check out Borderlands 1, as I don't think you have that one.

If you want long-lasting, go for an RPG. Witcher series especially.

Mount and Blade ?

Borderlands 2 is great. I've got 11 hours in at the moment myself and there's still tonnes and tonnes of content.

Kenshi, it's only in Alpha right now but it has a lot of potential and 10$ will be the cheapest price we'll see for a long time.

Witcher games. no question about it.

Fallout NV: Ultimate is $5. Witcher II is as well.

Rome and Total War will give you hundreds of hours of gameplay if you're into this kind of games

try bioshock 1 or 2 ^^ or both :P

Mount & Blade: Warband will go on sale for $5. It's an awesome game and will last you quite a while. I have friends with 400+ hours in it, I myself have around 100.

You should be able to puck up the base game of Civ5 for around $7.50, or less from trading. The game great for wasting time, I'll go for days at a time.

GTA 4 is also great to screw around in. If you plan on playing solo though, Sleeping Dogs is by far the better game. If you missed the bundle, Saints Row is also a must-have.

Dungeon Defenders, Binding of Isaac, Dungeons of Dredmor, Orcs Must Die! 2, Terraria

If you enjoy cRPGs, the HIB Weekly is amazing, 500+ hours of gaming for $4.56 (even the $1 tier will give you many hundreds of hours worth of gaming)

warZ or infestation survivor stories

If you like co-op, Trine 2

This one lasts for an hour and a half, good enough for ya?

If you can, wait until Sleeping Dogs goes on a better sale somewhere and get that.

If you're into adventure games, Testament of Sherlock Holmes. About 20 hours, give or take.

witcher 2

Kerbal Space Program. Might be a couple dollars more than $10 but it's the best money you'll ever spend on a game.

Give us a couple favored genres to put us on the right path.
From what I've played: the Witcher series is very good, Torchlight 2 is fun, you could go for a strategy game like something from the Total War series or Civilization; Dawn of War complete (that is DoW1+2+expansions) should be 10$ when -75%, which happened in the past, but the multiplayer population is getting lower; depending from the amount of friends playing, something like L4D2, Killing Floor, Natural Selection or even Dungeon Defenders could keep you busy for a while.

Fallout New Vegas. Doesnt even cost 10 bucks so you will have room for another game.

Mass Effect 2 and Borderlands GOTY