The what if machine. The possibilities of that thing are endless. Products could be safety tested without the need for human or animal test subjects. It could be used you to vet political candidates running for office. If you used it for personal use then you could run every possible life decision through it to find the beneficial outcome.
The only downside is that people could literally spend their whole lives asking themselves what if using the what if machine than actually living their lives. Very much like that black mirror episode that featured the device that allowed people to relive all their memories.