The Trojan War Deities did this frequently during the Trojan War.
The goddess Athena appeared in the form of a Trojan spear-man named Laodokos in order to induce another Trojan named Pandaros to break a truce between the Greeks and the Trojans by shooting Menelaus with an arrow.
Athena later assumed the form of Hektor's brother Deiphobos to delude Hektor, who was fleeing from Akhilleus [Achilles], to stop and turn to face Akhilleus under the pretext that he - Deiphobos - would help Hektor to contend with Akhilleus, which is how Hektor ended up dead.
The god Apollon [Apollo] took on the form of the Trojan warrior Agenor in order to fight against Akhilleus.
The sea-god Poseidon took on the appearance of the Greek prophet Kalkhas [Calchas] in order to encourage the Greeks when they lost a battle against the Trojans and were driven all the way back to their ships. Poseidon used this disguise because at this particular point in time Zeus had declared that none of the gods was to interfere in the fighting.
The messenger-goddess Iris diguised herself as the Trojan princess Laodike to convince Helen to go Troy's city gates to witness the single combat which Paris and Menelaus were going to fight over her.
In the Odyssey Athena frequently took on the form of Mentor, an old friend of Odysseus, in order to give advice to Odysseus' young son Telemakhos [Telemachus] while Odysseus was away at sea trying to get back home. It is from this relationship between the disguised Athena and the Ithakan prince Telemakhos that we get the English term "mentor" for a trusted, wise counsellor or teacher.
Seduction Through Deception
Centuries after Herakles had become a god, he appeared to the wife of his own priest Timosthenes on the island of Thasos, in the form of Timosthenes, and thus impregnated her with his son Theagenes, who became a famous Olympic athlete in the 500s BC. This is the same manner in which, centuries prior to this, Zeus had taken the form of Alkmene's fiancé Amphitryon in order to impregnate her with his son Herakles.
The war-god Ares appeared as a shepherd to the Arkadian princess Phylonome and had his way with her. She afterwards bore him twin sons named Lykastos and Parrhasios.
Massacre & Battle
The goddess Aphrodite once assumed the form of a Lemnian woman named Dryope in order to rouse the other Lemnian women against their husbands, who had taken other wives. This led to a massacre of all the men on Lemnos Island.
Hera, the queen of the gods, appeared to the Amazons as one of them to lie to them that her stepson Herakles had come to their country to steal their queen away as a trophy, which stirred their army into charging against Herakles to engage him in battle. Many Amazons were slain in the ensuing bloodshed.
The messenger-god Hermes disguised himself as a merchant in order to sell Herakles as a slave to the Lydian queen Omphale.
There's a pretty comprehensive list of the transformations in Greek mythology (including most if not all the instances in which a deity assumed the appearance of a human being) here: http://www.maicar.com/GML/METAMORPHOSES.html