Google is really dropping the ball with regard to surfacing Chromecast enabled apps in the Play Store. Sure, there's that Chromecast app page, but that's just a tiny fraction of all the apps and games with support for Chromecast now that third-parties have been turned loose. Enter Cast Store, which aggregates a bunch of apps with Chromecast support.
This is essentially a user-powered directory of links to Google Play. The app gives you the ability to search, read descriptions, and see reviews. Tapping on an app in Cast Store boots you into Google Play to download. The app itself is not bad looking, but there's an ad across the bottom of the screen. It apparently offers in-app purchases, but I have yet to spot them anywhere. Maybe you'll eventually have the option of getting rid of the ad for a nominal fee.
Cast Store has an option to submit Chromecast apps for inclusion in the Cast Store directory if they aren't already there. So do be aware this isn't a complete list of Chromecast apps – it's only as complete as the suggestions it receives. Until Google gets its act together, this is probably your best bet for finding a bunch of Chromecast apps in one place.
[Thanks, +Alexander Terry]