(http://img.photobucket.com/albums/v116/YellowYoshi/desktopdungeons.png)
It's Desktop Dungeons! (http://www.qcfdesign.com/?cat=20)
This highly addictive (and rage inducing... but you'll see that for yourself later) game plays like a miniature roguelike. Most single games take around 10~15 minutes. The basic gist is that you pick your race and class and get thrown into a single level map. You get to run around and collect items and spells, find a god to worship and serve, level up, and hopefully, take down the level's boss. There's tons of unlockables, including special maps and more classes and races.
Really, you will be addicted to this game once you've learned how it works.
(http://www.qcfdesign.com/wp-content/uploads/2010/03/screens_0_14a.png)
Experience the rage fun for yourself.
Oh wow. I really didn't deserve that win in the Snake Pit, but all I care about is unlocking stuff right now, I'll come back for a "real" clear of it later.
If you want to know a glitch you can abuse for fireball-reliant magical classes, find and worship Mysteria Annur, and learn the summon monster spell. Defeat all monsters of your level, and attempt to summon a monster. The spell will fail, causing no monster to be summoned, and no mana to be drained. However, Mysteria will still grant you piety for having attempted to cast the spell. Click the summon monster spell repeatedly to max out your piety, buy all of the available upgrades, and prepare to fireball everything in your path (you may want to max out your piety again afterwards, so it doesn't hurt if you start to piss her off).