The Minecraft Exchange
I would like to direct your attention to the Minecraft Exchange, an awesome Minecraft fan site which besides being a great source for all your Minecraft-related needs also holds building challenges every couple of days where you can compete against your fellow minecrafters!
From MCXC's About page:
"The Minecraft Exchange is mainly here to help spark inspiration by running regular contests, which we're calling challenges. We'll give you something to design inside (or maybe outside) of Minecraft, set a few constraints to make it interesting, and give you a few days to upload it here. For these first few weeks, we'll be picking the winners ourselves, but after we get rolling the community will pick the winners.
The Minecraft Exchange is also here to give you, the players, a place to show off your work. Have a cool castle that the world needs to see? Design an awesome skin for your blockdude? Write up a little app that does something awesome and handy? Upload them all here and share them with everyone.
Thanks for stopping by!"
So what are you waiting for? Go check it out!