We've written about AllCast before, which allows you to stream content from your Google Drive to a Chromecast. Localcast is an ad-supported, free alternative.
You'll need to add the Google Drive option after downloading the app to your Android device. Before you stream content, an ad will appear on your Android device. It's a little annoying, but if you don't want to pay for the full version (or the more feature-filled AllCast), it's a good free alternative.