What is it?
To quote http://kayin.pyoko.org/iwbtg/manual.php
, "This game is very hard. You could find that getting past the very first few screens is taking you the better part of an hour, or more. Do not be deterred. It's supposed to be unfair. Realize this before all else; any and all situations can kill without warning. Take nothing for granted. If it appears safe; you can be nearly certain that it is a trap. Understand that you can only get hit once before you die in a particularly gruesome manner. Be careful!"
In other words, it's a ridiculously difficult game. What makes it awesome is that all the levels and bosses are old-school video game references, and that the puzzles are very satisfying to complete. Not recommended for someone who gets frustrated easily (or at all, really, as you can die for no reason whatsoever, and have no hints at how to complete anything). If you get frustrated easily but are brave, make sure you play on Normal (default Hard), since that doubles the number of save points. Very Hard has no save points at all, I believe.
Also, since this game is similar to Cave Story in that it is sweeping the internet, is made by amateurs, and is really awesome, I want to recommend that as well, in case anyone here hasn't heard of it, and is into games like that (whee runon). Cave Story isn't frustrating, however, and is most often compared to Metroid in terms of gameplay.
So yeah. There you go: two awesome games for you people to play. Both 2D platformers, in case you're wondering.
PS Post here for help in IWBTG, since most puzzles/battles take 15 people to figure out.