Nu uh, no. All of you, I've got something to rock your asses off if you really want a challenge.

http://www.youtube.com/watch?v=NOTBy0lKSbo

Cloudberry, AKA "We Like it Hard"

Thank you very much for reminding me why I hate those games.

And while you enjoy your masochistic fetish I'm going to revel in my plattformer-free gaming experience.

Is that not bad for game design? Usually it must be possible for the tester to beat it.

Super Meat Boy, VVVVVV and They Bleed Pixels are all great.

*Cough*Konami Code*Cough*

For a raw fairchallenge Super meat boy hasn't been dethroned yet.

No it's not. I found it frustratingly difficult to the point where I had to ragequit a few times, but it's never unfair. 90% of the time when my character dies, it's because I panicked or got myself confused with the switching. I always found myself thinking "Screw this game," but never "This game sucks."

I also second VVVVVV. Excellent little game.

Cave Story+ and La Mulana are two of the best action-adventure platformers ever made!

lol forgot about that
Ah, it was just my personal opinion on the matter and I third VVVVV, that game is so fun.

Mark of the Ninja, excellent game. I don't know about realistic challenge but I thought it was alright in that department, not too easy or not too hard (my favourite).

Aztaka, there's a few game breaking situations (you can grind and find the right items) that makes the game very easy but still, the new version is rather good. There's a demo at Steam.

I bought Project Black Sun myself at GG, I thought it looked like a cool, castlevania-esque game but the save mechanics destroyed it for me. Does it get better later on? I don't want to waste my 2$ in vain. :P

Unepic, have you played it? Not exactly what I would consider realistic challenge but I guess it depends. It has some interesting humour (note: childish humour) and a weird setting.

I forgot to ask, is the new version of Spelunky available on PC?

It's ok man, I forgive you, update the kickstarter page after all :)

TowerClimb, a platformer with roguelike elements. Sort of similar to Spelunky, but different enough to be worth checking out. It's only in beta at the moment but still available for purchase.

Mutant Mudds.

Also, you definitely should check out They Bleed Pixels.