Today the first two demos were released for the Wii U eShop channel, Rayman Legends, which is about half a gigabyte, and Sonic & All Stars Racing Transformed, which is about twice that. According to someone at NeoGAF, you can play the Rayman demo 30 times and the Sonic demo 15 times, after which they will stop working.
The eShop doesn't have anything as useful as a special demo page or way to only search for games with demos, but if you look at a list of all the games (go to the bottom of the front page and choose "see all") you'll see next to the "more details" button an icon that looks like radio waves radiating towards a gamepad. This indicates that a demo is available. You can also do a search for the specific game to get to its demo.