What is a hallmark node in NXT (nxtcoin)? What is the purpose of a hallmark node for NXT?
From the NXT wiki


  A hallmark is a "stamp of approval" for a Nxt node.  By creating a digital signature based on your IP address and secret passphrase (private key), you are verifying that your account "owns" a node and is accountable for it.  This helps protect the network from attack, and increases the network's trust in your node. You do not need to generate a hallmark if you are a casual Nxt user.  It is a very good idea for those who operate public Nxt nodes on dedicated, public servers. Hallmarks have no impact on forging.