This app was mentioned in 7 comments, with an average of 0.86 upvotes
I've had one for about a year now (the Nexus Player). I love it, but it's not necessarily a replacement for your cable subscription. Officially it doesn't include any free content or anything like that, it just runs Android apps like Netflix, YouTube, etc.
I mainly use mine to stream media from my desktop PC. For this I use an app called Archos Video Player (https://play.google.com/store/apps/details?id=com.archos.mediacenter.video&hl=en) which has a pretty slick interface. Kodi (https://play.google.com/store/apps/details?id=org.xbmc.kodi&hl=en) should also work, but I haven't had any experience with it myself.
It's also a half-decent gaming console when paired with a bluetooth controller. I've been having fun with some emulators (including N64 and PSX).
I haven't had many complaints. The only one worth mentioning is that app compatibility isn't great, but it's getting better. Lots of apps which aren't officially compatible can be sideloaded onto the device, but any app that requires touch interaction will need a mouse plugged in to use (the only time I needed to do this was to configure my N64 emulator).
Archos video player buy the full version then add the codec pack which is free. Best video player I have used on ATV
https://play.google.com/store/apps/details?id=com.archos.mediacenter.video&hl=en
https://play.google.com/store/apps/details?id=com.m4rk3t.libcopy2&hl=en
Archos Video is better than Plex because it doesn't require a server to be running if you have a hard drive attached to your router. There is a free version but I gladly got the paid version because it's so awesome.
However, if you plan on only stream from your computer while it's running, you can look into Plex too. But Plex does require you run a server.
Archos Video Player might help you out. You can use your external drive, or network drive, and everything runs off the shield itself.
Try the Archos player, it works really well. https://play.google.com/store/apps/details?id=com.archos.mediacenter.video
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I could never get Plex to direct play anything 4k to my shield, this app does it perfectly.