Google has revealed a bunch of new apps that are now available through their Google+ Hangouts platform:

Google Art Project: Explore collections from around the world with thousands of artworks photographed in extremely high resolution. You can play an art guide and take your friends on a tour of different museums.
New YouTube App: Create, control, save and even share playlists of videos with friends—all inside a hangout. It’s like your own VIP table at the world’s coolest YouTube party.
New Effects App: One of the most popular hangout apps gets more fun with a brand new UI and with the introduction of new effects.
Symphonical: Conduct your business meetings and manage projects in real-time using the Symphonical app for hangouts.
MiniClip 8-ball Pool: Challenge friends to a game of pool with real-time stats tracking.
A Story Before Bed: Family hangouts just got better with this app. Pick a storybook from the shelf and read it to your loved ones.
Panoramio Game: Compete with friends to be the most precise wanderer by finding out where a photo was taken while traveling around the world with Google Maps through stunning Panoramio photos. If you are the best, you get to choose your next destination. Cheer On: Root for your favorite sports team by customizing your video appearance in a hangout. With so many sporting events coming up this summer, this is one app you must have next time you are in a hangout.

Google launched the API for developing such apps around three months ago. Developers have contributed more than 200 apps using this platform already.