This little gem has caused a flurry of activity over at 8chan, so I picked it up two days ago and have been pretty much playing it non-stop. It's an indie RPG developed by a very small team that has been described as "this generation's Earthbound." I can't really say whether that's accurate or not because I haven't played much of Earthbound (and what I did play of it I didn't enjoy), but if you're looking for a JRPG-esque game with a well-written story (composing of both great comedic and emotional moments), important player choices, "bullet hell" dodge mechanics, and that retro-spirte aesthetic, this game is for you.
In the world of Undertale, monsters and humans used to co-exist, until the humans declared war and sent all of the monsters deep underground, sealing them behind a great magic barrier. You play as a child who one day, while exploring in the mountains, falls down a hole into the Underground, where the monsters now live, and you have to find a way to escape.
One of the cool things about this game is that battling is a bit more open-ended than in most RPGs. You don't have to kill every single enemy (or boss) that you come across, because every single enemy in the game can be "defeated" without killing them. On top of that, there are consequences for killing or sparing enemies. Do you kill a tough boss so that he can't come back later in the game to fight you again? Or do you spare him and attempt to make friends?
The game also has a habit of breaking the 4th wall and making fun of JRPGs. It's really fun and I highly recommend it.