The Chromecast is already a pretty fantastic deal at $35, but less is always more when it comes to price. Both Best Buy and Amazon are offering the streaming dongle for $29.99. You still get the $6 credit for Google Play movie rentals from either retailer, so you won't be wanting for something to watch when you plug it in.
Google has been pushing the Chromecast hard as of late, adding a special section for Chromecast to the on-device Google Play Store to highlight the apps that can take advantage of it. Of course, those apps are still thin on the ground - outside of Google's wheelhouse there's only Netflix, HBO Go, Hulu Plus, and Pandora. If you're a regular user of these services, you shouldn't have any trouble justifying the purchase. If not... well, you could look at it as a $24 streaming gadget, assuming that you would have spent at least six bucks on movies.
Best Buy is offering free shipping, but in-store pickup is not available, so you probably won't see this small discount at any of their brick-and-mortar retailers. Amazon Prime members can get free shipping as well (and it's pretty cheap otherwise). There's no indication of when this sale will end. Chromecast could get a bit scarce around Christmas, so if you want a few for stocking stuffers, it might be best to pick them up now.