This app was mentioned in 10 comments, with an average of 4.60 upvotes
Imgur albums are very close on the roadmap.
Youtube videos though are only available through the official YouTube VR app for now (if installed, click on the "youtube.com" link next to subreddit).
There IS a VR version of YouTube, which is probably what you're missing. You can find it here
Once you have it installed, it should show up in your app list when in VR mode
You are going to have to be more specific on "Phone VR", are you talking GearVR, Cardboard, DayDream, ARKit? That last one I assume not, but you never know.
There is a legit Google app for YouTube VR supporting both Cardboard and Daydream modes.
https://play.google.com/store/apps/details?id=com.google.android.apps.youtube.vr&hl=en_US&gl=US
I've had spotty results with Youtube VR's 3D videos actually working. I think mostly because it's rather awkward to upload SBS 3D and 360 video. So unless the uploader really knows what they are doing, the final video doesn't display properly in the viewer.
There are lot of reasons why this is the case, but that is for a different thread. Suffice to say, keep trying various videos in the Youtube VR application and you'll eventually find some that work.
In addition, try out NextVR - There is a basketball game on there that is really immersive and they do a great job with camera positioning to make the best of depth. Also, as suggested by others, Within has a few good videos as well.
Google's VR app is NOT on cardboard. Get a clue before you argue.
https://play.google.com/store/apps/details?id=com.google.android.apps.youtube.vr
http://www.theverge.com/2016/11/10/13581046/youtube-vr-review-google-daydream-headest-best-app
Here's the official unmodified app https://play.google.com/store/apps/details?id=com.google.android.apps.youtube.vr
TLDW..
<strong>YouTube VR</strong> without a doubt, and not just for 360-3D videos (capped at mediocre quality right now anyway) but 360 spherical and 2D videos are great. I love pulling up trailers or Nat Geo featurettes in a private movie theatre with adjustable screen size.
<strong>Fantastic Beasts</strong> is a surprisingly impressive taste of the capabilities of mobile VR, and a neat little feature for Harry Potter fans.
<strong>Street View</strong> is supposedly the must-have app but I've found the experience too pixelated to be enjoyable. YMMV though.
Another fun thing is to use the <strong>Cardboard Camera</strong> app to take 3D panoramas (I love hikes and city streets) and view them later (in Google Photos) with friends. It captures sound, too, so you get some nice ambience and it's put a smile on everyone's faces when I show it to them.
The last no-brainer is Play Movies, which lets you stream or download for offline view—great for airplane rides this holiday season! :D (and apparently you can watch your downloads in YouTube VR as well, which I think has superior ambience and adjustability)
Not much right now, but several more—including games—coming in the next month or two.
Well for youtube, there's an official Youtube VR app.
For general web browsing, there's a bunch of apps that have browsers. A few that come to mind are DeoVR, Fulldive VR, VR Browser (which is a demo for 3dWebView for unity, but is still a functioning web browser), and i have my own app in the store called Home Theater VR, which has a browser, and can also stream from a computer.
Note that for the best web browser experience, you'll need a bluetooth gamepad.