Deus Ex the video game is quite a few years old at this point, but it still holds up to a playthrough every year or so - I tend to play it through, front to back, every Christmas. It's a hobby.
It's partially because of the plot, which is expansive - secret government agencies, military takeovers, The illuminati, public figures bent on taking over the world. I mean, hell, it's got everything. But also, the game makes it almost mandatory to challenge yourself. There are four difficulty levels, the only difference being the number of hitpoints you get. That doesn't sound like a big deal until you realize that the pistol bullet that stung on easy can kill you stone dead on realistic. This makes the first third of the game especially difficult - when you're first starting out your nano-augmentation abilities consist of a head lamp and that's it. No life regeneration, no shielding, no environmental resistances. If you get seen, you best get hidden before you get dead.
The AI in the game, while not all that advanced, is given a decided advantage over you in more ways than the sheer number of enemies you'll face. For example:
Let's say you're sneaking around a stack of crates when an enemy soldier notices you and shoots you with a poison crossbow dart. Assuming you were at full health (which is one hell of an assumption in the early stages of the game, but go with it) you've got a couple of problems. First, you've been poisoned, meaning your life starts to ebb away. You're also bleeding, which will stop eventually, but in the meantime will leave a handy trail of blood behind you wherever you go. And, because you're taking damage every second, you're going to be audibly grunting in pain every second - your enemies can, believe it or not for a game made over five years ago, hear you, and they'll come to investigate and, you know, shoot you more.
All of that's assuming you were shot once - if he managed to nail you twice, you're pretty much dead. You'll get used to that overhead spinning camera shot of your bleeding corpse right quick.
If you're playing the PC or Macintosh version of the game, you'll get another treat - some features were shaved off of the more ubiquitous console versions of the game so they could fit it on a PS2 CD-ROM. Some smaller areas were eliminated and some puzzles were simplified - the console version simply remembers what access codes you've found and inputs them when required, but the PC version, thanks to you having a keyboard, required you to type them in. Sounds silly, but it makes stealth operations a shit-ton harder when the doors don't just open for you.
That doesn't really affect the scenario I've laid out, but this change does: in the console version, you have a health bar and that's it. It hits zero and you die. Simple, right? Not quite - in the computer version, your body has zones of health instead of one statistic, so if your arms take damage your aim suffers; if your head takes damage you, well, die. But if your legs are injured...
That's right. You can't walk.
So in my example, you get shot (hopefully once), you're trailing blood, you're periodically (and loudly) yelping in pain and in five seconds or so you're not going to be able to run. And mr. badguy is taking aim for another shot...
As cool as all this is, it actually leads to one of the failings of the game.
Play long enough, and you'll get an upgrade that'll allow your health to regenerate. If you upgrade it all the way, you're practically invincible. Providing you don't blow yourself to smithereens (and there's an augmentation that takes care of that too, in case you were wondering, causing grenades to detonate practically before they leave their wielder's hands) you can pretty much tear ass through the levels with impunity.
That's where the self-challenging aspect comes in. After you've played the game through as many times as you can, doing things in different orders to get different rewards and dialogue results, you start setting conditions for yourself.
No health augmentations. No sniper rifles. No weapon upgrades. No scopes. Whatever floats your boat, really, because the game is winnable with these conditions. Fucking hard, but winnable. But there's a holy grail, a condition that gets you a place (in your own mind, anyway) among the true lovers of this game:
No fatalities.
The game was so ingeniously designed, it's technically possible to play through the entire game while only having to kill two people, Anna Navarre and Gunther Hermann, both mechanically augmented agents, both bosses, of course - it's interesting to not that, even though you have to kill them, you don't have to fight them: they have, well, self-destruct buttons of a sort.
Apart from them, it's possible to beat the game while only using non-lethal weapons - batons, tranquilizer darts, pepper spray, riot prods, gas grenades and a helluva lot of stealth. It also makes ammo conservation a problem. The lead game designer (whose name eludes me at the moment It was Warren Spector. Thanks Bitriot) called that his one regret of the game, that those two deaths were necessary. That was supposedly fixed in the sequel, though I've never played it.
It is, in short, a phenomenally great game. Anybody who gets a Mac OS X port together gets a big fat check from me.