Scythed chariots (known to the Greeks as harmata dreponephora, "scythe-bearing chariots") are a late and experimental development of the chariot as a weapon of war. From the late Bronze Age onward, chariots were used as weapon platforms; men mounted in chariots would wield spears, javelins or bows. The attachment of scythe-like blades is, to my knowledge, the only attempt to weaponise the chariot itself.
The earliest source ever to tell us about scythed chariots is the Athenian Xenophon. In 401 BC, he joined a large force of Greek mercenaries hired by Cyrus the Younger to try and wrest the Persian throne from his brother Artaxerxes II. The armies of the two rivals met at Cunaxa, near Babylon. According to Xenophon, Cyrus had about 20 scythed chariots, while Artaxerxes had 200 of them. They were drawn up in a long line in front of the heavy infantry.
The scythes they carried reached out sideways from the axles and were also set under the chariot bodies, pointing towards the ground, so as to cut to pieces whatever they met; the intention, then, was that they should drive into the ranks of the Greeks and cut the troops to pieces.
-- Xenophon, Anabasis 1.8.10
We don't know if the Persians were the first to put scythes on chariots, or when they first started doing so. However, the fact that Xenophon describes the weapon in this much detail makes it clear that his Greek audience had never seen them before. Xenophon himself clearly found them fascinating, and they feature prominently in his fictional account of the life of Cyrus the Great (the Cyropaedia). Given their stated purpose, it is possible that scythed chariots were a deliberate Persian invention to deal with the organised heavy infantry they faced in their frequent wars against Greeks and Egyptians. The purpose of the chariots was to crash into enemy formations, cutting off men's legs and throwing them into disorder as they tried to avoid the blades. At Cunaxa, however, they let the Persians down:
And before an arrow reached them, the barbarians broke and fled. (...) As for the enemy's chariots, some of them plunged through the lines of their own troops; others, however, through the Greek lines, but without charioteers. And whenever the Greeks saw them coming, they would open a gap for their passage; one man, to be sure, was caught, like a confused man on a race-course, yet it was said that even he was not hurt in the least.
-- Xenophon, Anabasis 1.8.19-20
Without a force to follow up the attack of the chariots, they seem to have been all but useless. When the Persians used scythed chariots against Alexander in a similar way at Gaugamela, they appear to have given the Macedonians no trouble either. The well-drilled pikemen were able to fall back locally and "box in" the chariots, at which point the charioteer could be easily dispatched.
Indeed, there is only one known instance of the successful use of scythed chariots in combat. During Sparta's war against Persia in the 390s BC, a group of Greek pillagers in Phrygia found itself confronted by the satrap Pharnabazus leading a Persian force of 400 cavalry and 2 scythed chariots. Greek disorganisation and light equipment meant that they could mount no effective defence:
When the Greeks saw him advancing upon them, they ran together to the number of about seven hundred; Pharnabazus, however, did not delay, but putting his chariots in front, and posting himself and the horsemen behind them, he gave orders to charge upon the Greeks. And when the chariots dashed into the close-gathered crowd and scattered it, the horsemen speedily struck down about a hundred men, while the rest fled for refuge.
-- Xenophon, Hellenica 4.1.18-19
Perhaps because of this engagement, and others that we don't know about, the Persians actually stuck with the concept of scythed chariots until the end of their empire - and the Hellenistic kingdoms continued the practice. Scythed chariots are still listed as one of the branches of the generic army sketched in the tactical manual of Asclepiodotus hundreds of years after the death of Alexander. At this point their novelty - like that of war elephants - had well and truly worn off, and unlike elephants the scythed chariots had no major successes to their name. Possibly they were retained more for their fearsome appearance than for any practical purpose in battle.
They were used historically, as far as we know from ancient sources, but seemed to have been very ineffectual. The problem was weight, control and direction. They could be easily slowed or stopped by rough terrain, they could be bogged down easily because of their weight, and the horses could be distracted or re-directed away from their target fairly easily (The recent film of Alexander the Great shows this quite well.). They were occasionally effective, however, in one battle, the details of which I can't quite remember, but was between a Seleucid or Pontic army, both of which used them, fighting one of the Roman Asian client states, they were able to charge into a phalanx with devastating effect. Most writers though describe their failures. Xenophon and Arrian describe their use by Persia, Polybius, Livy and Frontinues describes their failures in use by the Seleucids. However, these are the only seriously accepted use of them. The descriptions of their use by Britons and Gauls is almost invariably incorrect translations of the latin, or the acceptance of poetic language as reality. There is no archaeological evidence of scythed chariots in the west, but there are examples of what could be scythed chariot parts on the carvings of trophies at Pergamum.