So in light of the latest chapter (628), we see a new Poneglyph discovered; and that makes me think when was the last time we saw any new revelations about it. Answer: chapter 395 in Ohara, and I had to search my chapter collections to find it.
So I was thinking, wouldn't be nice if we had a thread to record it. So I decided to make one, as thorough as I can at that. I hope its enough to spark a discussion, but if not at least its good enough for reference (I hope)
So, without further ado:
1. The first mention of Poneglyph is at Arabasta, located in the Royal Tomb.
! >! 
Taken from chapter 202 pg.18 and chapter 218 pg. 03
! Revealing the existence of Pluton and whereabouts, though the latter is unspecified here
2. Later was at Skypiea, located at the base of the Golden Bell
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Taken from chapter 301 pages 11-14
! Poneglyphs' Translation:
Keep thy motives in heart with closed mouth.
We are those who control history, with the tolling of the belfry.
An "ancient weapon" with the name of a god. "Poseidon"… and its location.
! Robin's Facts:
There are 2 types of Poneglyphs: Tablet of Clues and Tablet of Truth, with the first leading to the 2nd.
The Skypiean Poneglyph is a Tablet of Truth
And the True History will be revealed by combining all of the Tablets of Truth
With the recent chapter (649), Poseidon was revealed to be an ability to talk to, and call forth Sea Kings. An ability born every once in some-hundred years. And now Shirahoshi is the owner of that ability
3. Ohara Scholars, with Prof. Clover revealing their findings and conclusions regarding those Poneglyphs
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Taken from chapter 395, pages 06-08
! Prof. Clover's Theory:
Clover: Perhaps. Or perhaps it suits YOUR needs to make things thus!!
What we want to know most of all at this point in time,
is not the contents of the Poneglyphs, but their reason for existing.
Why did the people of the past use stone monuments,
to leave some message for the future…
They carved history into unbreakable stone and spread them around the world...
Is it not because their message could be destroyed on mere books and paper?!
Clear proof that the people who left them had ENEMIES!!
! Clover: If we assume those people were wiped out by some "enemy,"
it means those enemies should have survived later in history.
Coincidentally enough...
800 years ago, at the end of the Lost 100 Years,
was the precise birth of today's "World Government."
If the enemy of those eradicated peoples is our World Government,
then the "Lost 100 Years"
could easily be a piece of INCONVENIENT HISTORY that the World Government needed to stamp out...!!!
From the ancient writings of the few Poneglyphs we HAVE found,
we have learned of the existence of a nation... One which no longer exists in any form,
but still appears in those messages.
An ENORMOUS KINGDOM...!!
! Clover: They seem to have held great power once,
but the information about this kingdom has been carefully erased.
As they knew of their imminent defeat by the alliance
that would later be called the World Government,
they carved the truth into stone, to give to the future.
Those are the Poneglyphs that still exist today...!!!
! Clover: The Ancient Weapons are truly a threat to the world!!! BUT!!
More importantly, the existence and ideals of this country,
that threaten to be brought to light again are the GREATEST threat to your World Government!!!
I will not know what threat this is until it is revealed,
but the name of this once-mighty kingdom,
which holds all the keys, was--
4. Fishman Island, located somewhere in the Forest of the Sea
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Taken from Chapter 628 page 15, and Chapter 649 page 16-18
! Robin's Facts:
This Poneglyph is different in content from the rest. It was actually more of an apology letter from "Joy Boy"
! Neptune's Explanations:
The Poneglyhp was an apology letter from Joy Boy (a certain man that lived during the Void Century) towards Poseidon (the Mermaid Princess of that era, the wielder of Sea King summoning ability)
The apology was due to Joy Boy's inability to fulfill his promise, the details of that promise wasn't known
But the promise involves Joy Boy, Poseidon, and the ship Noah (which was made to be pulled by Sea Kings). And the promise will eventually be fulfilled by someone else, someone will guide Poseidon and make use of the ship and the Sea Kings to change the world. So Poseidon itself is both the name of one of the Ancient Weapons and the name of an ancient mermaid princess. Hence, by having the same power as her predecessor, Princess Shirahoshi is now the current Poseidon.