Dona Room is a game that has been around for just a few weeks, and yet I have received many suggestions to include it here. It is another of the 'escape-the-room' variety of point-and-click Flash adventures, and it is very good. As with other games like it, look around the various objects in the room to collect items and find clues on how to escape.
The game was created by Naoki Nakashima, and contains a small amount of Japanese writing that explains the story behind being stuck in the room. These details are not necessary to solving the game as all of the major clues are given as symbols and Arabic numerals (1,2,3...).
A couple of hints to save you some frustration: in a world of symbols, a filled symbol is not the same as a non-filled one; and a triangle pointing up is not the same as one pointing down. Also, you must find all items and view all of the clues before the safe will open—even if you enter the correct code. When you finish the game, be sure to read the comments to learn what all the numbers mean, as Maiko has so generously provided a translation of the game's amusing ending.
Thanks go to Karina, Evan and everyone else who suggested this one. =)