Recommended games for Steam?

Plants vs Zombies first and foremost. Otherwise, well, depends on your genre preferences I suppose. My Steam collection has a lot of classic adventure games in it.

Also, for Arx Fatalis, food for thought: Good Old Games sells it for 6 bucks, and their version is entirely DRM free. You download it an unlimited number of times and it works without any form of copy protection.

If you like rhythm games, Audiosurf is never a bad choice. It builds tracks from your own mp3s.

RPG's are my main genre, but my tastes are eclectic. I'll play anything short of sports games, especially anything unique or anything that requires a good amount of thinking.

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Half Life
Portal
Painkiller
Serious Sam HD
Mount and Blade- Warband
Left 4 Dead (1 and/or 2)
Audiosurf
Recettear

Those are the basic essentials. The rest of your soon to be massive game list will whatever tickles your fancy, or whatever is at a ridiculously low price that weekend.

Recettear is a ridiculous amount of fun.

Why aren't more people playing it?

What type of games do you like? There are literally over 1,000 games on Steam so giving a recommendation might be difficult.

He says he likes RPGs. I would go with something in those lines. Deus Ex was a good choice.

Is Viewtiful Joe available on Steam?

//Zig

RPGs on Steam off the top of my head:
Deus Ex
Fallout 2
Fallout 3 GotY
Vampire the Masquerade Bloodlines
Torchlight
both Freedom Force games
Oblivion
Borderlands (if you have a friend to play with)

For non-RPG's Alien Swarm is decent and free. Also get Orange Box, Plants vs Zombies, Left 4 Dead 2, Half Life, Garry's Mod, World of Goo, at least 1 Total War game (Rome and Empire are my favs), Civilization V, list goes on...

I also played Arx Fatalis, decent game and kinda challenging but also kind of bland and boring.

There's also Puzzle Quest 1 and 2, Puzzle Quest Galactrix, Puzzle Kingdoms and Gyromancer if you're into the whole puzzle RPG thing that exploded after the original Puzzle Quest came out.