Enchanted Coin

Fake coins with the Protean Charm cast on them were used to set up meetings for the DA.  Also, a similar fake coin was used by Draco Malfoy to communicate with Madam Rosemerta while she was under the Imperius Curse.

Hermione was able to cast a Protean Charm on the fake Galleons the D.A. used during the 1995 - 1996 [Y15 - Y16] school year to pass along the date and time of each meeting. The spell caused the numbers on the members' coins to change when Harry changed the numbers on his coin. The Ravenclaws were all astounded that Hermione, who wasn't in their house, could cast this spell, since it's N.E.W.T. level. Clearly none of them could cast this magic at fifth year level (OP19). Ironically, Hermione got the idea from Voldemort's use of the Death Eaters' Dark Marks as a means of communication.

Draco Malfoy, in turn, deliberately copied the notion of Hermione's D.A. coins to enchant coins as a means of communication for himself with Madam Rosmerta (HBP27).

Once the second war had broken out, with some of the D.A members still at Hogwarts and others spread out all over the place, (and with everyone being watched), Neville was still using the fake Galleons to communicate with everyone.  (DH29)