6EQUJ5

I'm not a clairvoyant and certainly don't own a time machine, but I'm pretty sure that as soon as humans (or their ancestors down the evolution tree) were able to not only admire the night skies but also their immensity, they must have wondered if there was anything or anyone else out there (cue in gods, angels, ghosts, or "fireflies stuck that, uh...got stuck on that big bluish-black thing").

And, of course, as science evolved as well as humans (assuming everything evolves linearly), so too did our concepts of what could be out there, including other inhabitable planets with little ETs roaming about and pointing at each other with their glowing red-tipped indexes. Which spurred a whole new form of scientific research--that for life outside of earth.

It is on such a quest that, in the summer of 1977, a volunteer for the Search for Extraterrestrial Intelligence program named Jerry Ehman, recorded a signal from outer space. The radio signal (or the "Hydrogen frequency" for those engineers out there who are prickly about details), coming from the Sagittarius constellation (near the Tau Sagittarii star to be precise, about 220 light years away), lasted 72 seconds, and was recorded as '6EQUJ5'--circled in red with a 'Wow!' beside it.

The sequence described here "describes the intensity variation of the signal" from 0 to 10, then letters for anything higher than that (with 'U' standing for 30-31 standard deviations above the mean of the background noise caught by the antenna). People were even more excited about this signal noting the fact that it matches the hydrogen line and it's believed hydrogen is the most common element in the universe so aliens could use it to transmit signals.

However, the signal has never been repeated and scientists have speculated that if the signal had been transmitted by extra terrestrials, it would have had to have come from a highly advanced civilization that would have used a 2.2-gigawatt transmitter. In the meantime, the Arecibo Observatory sent out a responding signal in outer space containing information from 10,000 Twitter feeds (I shudder to think what any ET would think of us if he/she/it were to receive and decipher it!).

Arecibo message:
(1) numbers 1 to 10
(2) atomic numbers of elements H, C, N, O, P
(which combined make up the DNA)
(3) formulas for the nucleotides of the DNA
(4) the number of nucleotides in DNA + double helix
(5) human figure + avg. height of man and tot. population
(6) solar system
(7) Arecibo telescope and transmitter antenna dimensions

Sources:
Seti League article
Wikipedia
Discovery News