The Katelabs Advent Calendar! 25 doors each containing a small world makes this (I believe!?) the biggest Katelabs world to date!
FAQS:
Q: it runs so slowly! A: get the most recent version of Katelabs, where I sped the game up kind of specifically to handle things like this!
Q: can I open all the doors at once? A: yes, but if you do you have to leave a comment as a trade.
Over the past ten years I've been making lots of tiny games!
This year for the run up to Christmas I took 25 of the best ones, made sure they worked on modern browsers, and put them here: http://games.alienbill.com/advent/
One is unlocked everyday. (or if you know ANYTHING about javascript, you can hack your way and see them all now.)
I usually use Processing, but GloriousTrainwrecks was a huge inspiration, and a lot of these are from various KotMs and events here