Almost 30 years ago, I was bored one evening and decided that what I really wanted was a D&D-like game I could play by myself. So I slapped together a little boardgame called DeathMaze that SPI published a year or two later. Since then, there have been probably hundreds of similar games published -- indeed, even from the inception of digital games, with titles like Wizardry. At present, my favorite game of the genre is FastCrawl, which is a nicely polished version; but "free" is a nice price to pay, and Monster's Den isn't bad either.
It's the usual thing: Select your starting characters and enter a randomly-generated dungeon. Combat is turn-based, with the usual front row/back row arrangement, but no real positional importance beyond that. There are unlockable skills, and loot, of course. Simple, straight-forward, mildly fun; no great surprises, though, and it does seem that the earliest levels are the most challenging, which is a pity.
Not a bad way to kill an hour or two.