InsaneDifficulty.com is a community site dedicated to modifications of classic games which make them more difficult and complex. There are many games hosted by the community: Chrono Trigger, Super Mario RPG, Final Fantasy VI and Dark Souls among others. Most mods are supported in dedicated forums (including installation instructions) on the site's message boards.
Although the Creation Kit mod development platform for Skyrim (previously) is still a few days from being released, intrepid modders have been working with edited game files to achieve mixed results. One of those results is the "Macho Dragon Mod" (download link). [more inside]
Bike Hacks! Bored with your generic two-wheeler? Check out this collection of funky bicycle modifications. My favorite is the grocery cart.