If you want a game that truly challenges you, this game will be the one that answers the call while demanding you put your back into it.

SpaceChem is basically making a factory line on an atomic scale; build a structure around bonding, breaking and creating molecules. This concept is not only simple, but also versatile and effective at challenging you at every turn. Don't be suprised you'll scratch your head twice on a few levels; the handholding is very quickly dropped.

All in all, this game is not for the simple-minded, so I recommend it for the ones that look for a truly mindbogging experience.

TF2 is pure proof of multiple things that are known about Valve; They know how to make a colorful, fun, non-repetitive and challenging game.
Being released in 2007, it ages quite good. Even after 4 years, it never fails to deliver a hectic, chaotic and addictive experience to anyone playing. It's alose still being updated at least each week.

A small, but fair warning, however; Over the years, Valve added more and more accessory like hats, weapons, items and even a shop to buy them (but mainly hats), watering the original style down.

With that out of the way, I recommend TF2 to anyone, since, well, it's for free nowadays. Keep playing if you like it, delete it if you don't. Your choice.

Dogfighter is not your average Flight Simulator. Heck, I doubt anything like it exists here on Steam. Controls can be a bit of a chore in the beginning, but with the right tweaks you can preform the most wicked of acrobatics.
Unlike other games with the same concept of "You fly around and just shoot things"-idea, Dogfighter is suprisingly fun to play. Will it be the versatile maps you fly around in, the sometimes badass powerups or the neat tri-, bi- and monoplanes at your disposal, Dogfighter can be a hectic, challenging and most important, fun experience.