Anyhow, for the lucky third week:
Game type: Downloaded. Simple undemanding-of-resources window-based game.
Time demand: In theory, since you save as you like and play whenever, it's however much you want to put into it. In practice, chances are good of unintentionally plunking a couple hours at a time into it.
Cost: Free, always.
What it is: C'mon, it's Nethack. For those who haven't played it before, it's a game where you play as an adventurer exploring the Dungeons of Doom, with a specific goal: collect the Amulet of Yendor and take it to the heavens to sacrifice it on your personal god's altar. And believe me, it's not at all simple.
You can save as you like, but the catch is this: saving is only basically in order to stop playing when necessary, and start up again when you like. Your saved game is wiped out whenever you load it, so if you quit without saving, or die, there's no save game to go back to. (Unless you cheat and copy the file to another directory, which is considered Really Bad Form to put it mildly by Nethack players everywhere.) The entire goal is to get through the dungeon without dying. Or at least, without dying when you don't have a backup like an amulet of life-saving on.
And this is very very difficult. There are people who have played for years and never managed to Ascend (er, win the game).
The bare-bones version of the game looks extremely simple in graphics, if you can even call them that - it's ASCII characters that represent various things. The walls and tunnels of the dungeon are formed with | and # characters respectively, your own character is a simple @, and various other letters and symbols represent the dungeon monsters, people, and objects. There's also a tiles version of the game which replaces the simple ASCII characters with simplistic pictures, making the game... er... still extremely simple in graphics, but not as much so as the original.
You can play as one of various class and race combinations, with different races being able to be different classes. Each class has its own gods, chaotic, neutral, and lawful, and you follow one of them. Each class also has a completely different playing style, making a trip through the Dungeons of Doom with a new class and race a unique experience, which ups the replayability factor. Of course, said factor is already through the roof, because each time you play, the dungeons are randomly created, with some restrictions. (Always an entrance to the Gnomish Mines on levels 2-4 for instance, certain levels always appear somewhere in a certain range, monsters can only be of a certain level depending on dungeon depth and character experience level, etc.)
There are also a ton of things that make this game both unique and enjoyable. There are many different object interactions, some of which are not immediately obvious (such as using a wand of turn undead on either an undead monster or a corpse for... quite different results). And of course there are geek references throughout the entire game that add to the fun factor: such as if a elven-race character is about to starve, instead of the normal hunger message, you get "Elf needs food, badly!"... or the whole Tourist class in general, which is a nod to the character of Twoflower in the Discworld novels.
I could go on and on about the game, how to play it, and all that, but this entry is already more than long enough. So I'll just wrap this up by saying: if you haven't played this before, do so! Now!
Edited to add: I did somehow totally forget to mention that this is a roguelike game - one of the original ones that spawned the genre in fact. So if you've played Diablo, Azure Dreams, Chocobo's Mysterious Dungeon, Etc etc fricking etc., you have some experience with this type of game, and some idea of whether you'd like it or not. Which you should. Or else you suck. :p