This app was mentioned in 4 comments, with an average of 99.50 upvotes
To summarize:
Mobile Passport should not be confused with Global Entry, they are two different programs
Who are eligible: US citizens with valid US passports, Canadian citizens with valid Canadian passports
Where can you use this app (when landing): Atlanta (ATL), Miami (MIA), Seattle (SEA), Chicago (ORD), San Francisco (SFO); more airports TBA
App generates a time-sensitive QR code with a four-hour life, to be scanned by a CBP officer
Upvote because I'm also interested in this. Ever since they released their official Mobile Passport app, I've longed for a world with no queues at customs (ಥ﹏ಥ)
You can also do mobile passport. Almost as fast. https://play.google.com/store/apps/details?id=us.mobilepassport