Well some of the railgun tech we currently have, HAARP and the like (if it is actually what they say it is, which is up in the air).
World leaders would essentially lose their grip over the power of the people. Why would people respect them (more than they already do) if it turns out that they've spent two thirds of a decade disclaiming the existence of beings we all concluded were real? Moreover, many people would denounce the leaders in favour of more technologically advanced creatures with a sounder understanding of the working universe. Entire religions could potentially be torn apart if these beings could prove what happens after death (something many abductees and ufologists corroborate) or that there is/isn't a creator. Science as we know it could be revolutionised, but our understanding would be limited by our own human brain, leaving us frustrated at what we aren't capable of understanding. Society would be chaotic, what would be the point in going to work or following the rules if it turns out after death we just go through it all again, regardless of how we die? Or, if the aliens were benevolent yet orderly, they may instigate new rules and laws for us to live by.
It would vindicate everything ufologists, paranormal investigators and other people in that field have worked tirelessly to prove, but it would come at a great, great cost to us all.