Jetpack Joyride and Rampage
There are a lot of fake game announcements going around today, but Riot has won the day for me by making its retro idea into an actual game. CHEW - Cho'gath Eats the World is an awesome little game where the goal is to smash the buildings not unlike Rampage. Astro Teemo is basically Jetpack Joyride with a trippy Teemo remix happening in the background.
The soundtracks alone makes them both worth taking a look at. Hopefully Riot keeps them available for more than just today, but just in case you should all go check them out. If you have trouble getting either game to run, try using Internet Explorer.