Sometimes you just want a good, brainless dungeon crawl. Monsters' Den lets you put together a party of four characters (chosen from the requisite fantasy classes: warrior, cleric, mage, ranger, rogue) and then head into the dank, randomly-generated dungeons of wherever to do battle with all manner of grisly monsters.
The game mechanics are very simple – just click around the map to explore new rooms, collect treasure, and engage the denizens of the dungeon. Combat is tactical in the sense that your party's formation (set before each fight) determines who can hit or get hit by whom, but overall it's pretty fluffy (which isn't necessarily a bad thing). After fighting, you get to collect the requisite loot, which is never in short supply when you throw in all the treasure chests lying around – after level one you'll have a lion's share of interesting weapons and armor.