Dwedit posted this game on another forum and felt compelled to post it here (and a bunch of other places as well). It's just that good.
Game website:
http://zarat.us/tra/offline-games/eversion.html
Direct download link:
http://zara.verge-rpg.com/eversion.zip
Controls:
esc - Go to title menu (you can revisit any stage that you've played)
p - Pause
z - Jump
x - "Evert"
The goal of the game is to "rescue the princess" (yawn), but the real challenge lies in getting all of the gems. The main draw of the game is the ability of your character to "everse" between different worlds (sorta like Zelda 3) when at a specific spot. You can tell you're at an "eversion point" when the background color changes. The differences between the worlds are not only cosmetic, but also effect things like block properties and enemy placement, making the game sort of puzzly. Eversion also changes some other things, but I won't spoil it for you.
Just a note of caution: To me, this game is very reminiscent of Sunsoft's NES titles, in that it's deceptively kid-friendly exterior hides a nerve-racking and nightmarishly difficult interior. I found it to be quite short (only 7/8 levels) and manageably difficult, but I know that some of you won't have the nerves to complete this game (things start heating up about halfway through). Still, I found it fun, but it's definitely not for everyone.
Have fun! :-)