For all the issues with the Duo Camera on HTC's flagship One M8, the selective focus thing is pretty fun to play with. With the newest update to the HTC Gallery app in the Play Store, M8 owners can share Duo Camera images to the web that anyone can use to change the focus. It's sort of like those Lytro camera embeds. We have one below you should try out.
HTC's Ufocus editing tweak uses the depth information acquired by that secondary lens on the back to generate a blur effect, making it look like you took it with a nice lens with shallow depth of field. It's a software trick, but it's more reliable than most similar solutions on other devices. In the updated Gallery app, just share your Ufocus edit and you can embed the image anyplace online. It's also hosted by HTC.
Click around in the image above to change the focus. It's pretty cool, right? Wherever you click, the image will "refocus" there. Of course it's just software, but at this resolution the imperfections aren't as noticeable. If you save the image, it preserves the focus you selected (not at full res, though), which is actually awesome. HTC really should have had this working when the M8 launched—it might have convinced more people to buy it.
Update: We've been digging around and have learned a little more about how HTC is doing this. The image is dumped into your Google Drive, but it's actually split into three different re-focused versions (0, 1, and 2). When you click on an area to focus, the embedded object pulls up the right version of the image. That's how you are able to save a regular jpeg. You may actually notice the file names are 0, 1, or 2 when you save them.