Some interesting stuff to do online
Virtual NES
It has a lot of NES games that require no downloading at all. Most of you might already know this, but I dont think EVERYONE does, so...
This one is not actually 'interesting', its just kinda useful, assuming you still play minecraft.
Minecraft Recipe List
Not just because it has all the recipes, but actually because it tells you the very last update/snapshot for the game at the right of the screen, so its a good thing to have in your bookmarks.
Seaquence
This one is actually pretty cool. You can create tunes by customizing some little creatures that resemble a microscopical life form. Each one produces a determinate amount of notes that you choose, and some other things. It's tricky until you figure out how to customize the notes just like you want them, but its cool to learn as you play.
Aannd..
Incredibox