If you have never even heard of this slightly old Indie game then where the heck have you been? Youtube it, especially the vids by Cloud8745 and his famous rages, I'll link to the first part below if you wish to watch before checking out the link to the official IWBTG site.
Basically this game is designed to make you rage like none other and push your skills (and patience) to the very limit and beyond with countless deaths. This game is NOT for the casual gamer as you will find yourself wanting to do many bad things in anger if you rage even the slightest bit.
However if you wish a game to test your skill at, then "I Wanna Be The Guy" is for you.
The person who really helped make the game famous with his youtube commentary I present part 0 (yes there is a part 0) of Cloud8745's videos which are now uploaded by other users at his permission since for whatever reason I forget they were taken down ages ago. This user has the entire playlist.
And remember...I wanna be the guy!
Also fair warning....excessive swearing is in all of the videos but they are allowed on youtube without the maturity filter login required so take it how you will.