If you've never heard of this game, close the thread before you're sucked into one of the most addicting, least graphically attractive games in existence.
Have you ever felt the need to build a castle block by block out of soap processed from the rendered fat of a thousand puppies?
Ever had an urge to encase a troop of Elven merchants in Obsidian created by mixing a large amount of water and magma?
Have you ever wanted to play a game in which a thriving 200 dwarf fortress can be turned into a smoldering hellhole of death and anguish in mere minutes?

Latest version is 0.28.181.40d.
Latest version of tweak is here.
Important Sites
Dwarf Fortress Wiki
Dwarf Fortress File Depot
Dwarf Fortress Development Log

Dwarf Fortress is a character-mode game that has two distinct styles of play:
Fortress Mode
Control a band of plucky, enterprising dwarves and build up a home for them; manage construction, build workshops and watch tiny dwarven drama unfold. Sort of like Sim City, but with more detail and undead groundhogs. Not at all rogue-like.
You begin with seven dwarves and a waggon full of supplies. What you do from there is entirely up to you, and with each new release there's more to do.
Here's a screenshot, showing a fort's entrance. Two goblins have tripped stonefall traps, killing them. A trade depot full of goods was just out of their reach.

Adventurer Mode
Here is your rogue-like. The commands are much different than others in the genre. Gather adventurers, buy equipment, read inscriptions and slaughter goblin children.
Adventure mode features a very in-depth fighting system, where you can really tear things up in amazing detail. You will also die in amazing detail, probably to monkeys. Or fish. Or hippos.
Part of the joy of DF as a whole is creating fortresses in fortress mode, then exploring them in adventurer mode. You'll be able to get a whole new perspective on how your efforts as an overseer went in adventurer mode.

Dwarf Fortress is one of the most open-ended games of all time. How many games do you know where you can watch a goblin try to steal a finely crafted crown, only to have his left kidney torn out by a wardog while a wrestler tears his arms off?
If you're still skittish about playing this game you should read the Epic Tale of the fortress Boatmurdered. If this doesn't suck you in, it might not be your game.

The Dwarf Fortress Wiki is a great place to start.
Try Indecisive's Illustrated Tutorial
Come talk to other dorf-nerds on irc: irc.synirc.net/#df
Very cool video tutorial
Dwarf Fortress Video Tutorial part 01 - World Generation & Finding a site: Youtube HD
Dwarf Fortress Video Tutorial part 02 - Preparing the Journey: Youtube HD
Dwarf Fortress Video Tutorial part 03 - Understanding the DF world & Naming Dwarfs: Youtube HD
Dwarf Fortress Video Tutorial part 04 - Mining and making beds: Youtube HD
Dwarf Fortress Video Tutorial part 05 - Making Bedrooms, farms and stairs: Youtube HD
Dwarf Fortress Video Tutorial part 06 - Dining rooms and dealing with refuse: Youtube HD
Dwarf Fortress Video Tutorial part 07 - Brewing, labor options and dumping: Youtube HD
Dwarf Fortress Video Tutorial part 08 - Bedrooms, crafting, trading: Youtube HD
Dwarf Fortress Video Tutorial part 09 - Crafting and Hotkeys: Youtube HD
Dwarf Fortress Video Tutorial part 10 - Migrants, Military and Requests: Youtube HD
Dwarf Fortress Video Tutorial part 11 - Requests, Training and Specialized Stockpiles: Youtube HD
Dwarf Fortress Video Tutorial part 12 - Migrants, Military and Requests: Youtube HD
Dwarf Fortress Video Tutorial part 13 - Trading, Migrants and Defense: Youtube HD
Dwarf Fortress Video Tutorial part 14 - Moods, crossbows, engraving: Youtube HD
Dwarf Fortress Video Tutorial part 15 - Forging part 1: Youtube HD
Dwarf Fortress Video Tutorial part 16 - Forging part 2, Fortifications: Youtube HD
Dwarf Fortress Video Tutorial part 17 - Butchering & Cooking: Youtube HD
Dwarf Fortress Video Tutorial part 18 - Leather & Wells: Youtube HD
Dwarf Fortress Video Tutorial part 19 - Ambush & Coffins: Youtube HD
Dwarf Fortress Video Tutorial part 20 - Siege & Traps: Youtube HD
Dwarf Fortress Video Tutorial part 21 - Nobles & Managing: Youtube HD
Dwarf Fortress Video Tutorial part 22 - End of Fort 1 & 3D view: Youtube HD
Dwarf Fortress Video Tutorial part 23 - Start of Fort 2, Magma Pipe, & Underground river: Youtube HD
Dwarf Fortress Video Tutorial part 24 - Dragons & Sieges: Youtube HD
Dwarf Fortress Video Tutorial part 25 - Water channeling & U-tubes:Youtube HD
Dwarf Fortress Video Tutorial part 26 - Flood gates, Levers & Bridges: Youtube HD
Dwarf Fortress Video Tutorial part 27 - Magma forging, Roads, & Ponds:Youtube HD
Dwarf Fortress Video Tutorial part 28 - Glassmaking:Youtube HD
Dwarf Fortress Video Tutorial part 29 - Adamantine & BAD END: Youtube HD
You start off by generating a world (or downloading a premade one). The World Generator has been drastically updated. You can choose to create 10 or 10,000 years of history for your world, how volcanic it is, how evil (or cheerful) among many other things, and have it evolve before your eyes.
You'll see changes to the world as habitats change and interplay; forests spreading out in a rush of green, or deserts rolling over the grasslands and stripping the land bare. The world's inhabitants spread out across the globe in a struggle to survive in a living world of dangerous environs.
I recommend reading about The Updated World Generation, even if you've played previously.

It's an alpha. Enjoy! Here's a big list of bugs to watch out for, along with those that have been fixed.
This game runs slowly.
There's a lot of stuff going on in DF at any given point in time, and it can experience slowdown for a few reasons. For hints on boosting performance, see Maximizing framerate.

Here's the big list of Character Sets
And the big list of Tilesets
Character sets are a taste issue, but Dystopian Rhetoric and Mike Maydays sets are popular.

Rick has consolidated his collection of programs into Tweak. A glory of modern programming, Tweak allows people to write modules that affect the game in many ways. The default tweaks are:
Adjust Profile - Allows editing of the skills for starting dwarves in the prepare screen.
Adjust Start - Allows editing of starting dwarf count and starting points.
Heal - Heal or hurts a creature.
Lookup Addresses - Pattern scanning for finding addresses in new versions of Dwarf Fortress.
Reveal - Reveals all allocated tiles on the map.
Tile Edit - Edit nearly all properties of a selected tile.
Warp - Warp a creature to another location.
Keep in mind, with the new version just released, Tweak is not working. It takes a while to work these things out and it will be back up soon.
There is another tool for visualizing your fortress in 3D. 3Dwarf is a pretty amazing tool, and is pretty fun to use.
Finally, for all your dwarf fortress mod and tool needs, The Dwarf Fortress File Depot contains ever file known to dwarven kind.