The next major upgrade for Ethereum was initially code-named Metropolis, but due to its complexity, it was further broken down into two stages, code-named Byzantium and Constantinopole.
The first stage, Byzantium, was successfully executed this morning. As you can see from this tweet below, the devs, including Vitalik Buterin, are celebrating it.
At the moment of writing, there were still clients which weren't updated, but according to the devs, this will not lead to another fork within the network.
If you run an Ethereum node, then you should upgrade from Ethereum Github repository.
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