What reforms did Julius Caesar bring?

Julius Caesar's most enduring reform was the calendar. He switched from a lunar to a solar calendar. Apart from some minor changes introduced by Pope Gregory XIII in 1478, this is the calendar we still use today.

Caesar introduced a land reform to distribute land to the poor and to retiring soldiers. He created a police force. He passed a law which wrote off 1/4 of all debts. Another law rewarded families with many children to encourage the re-population of Italy. He put a limit on the purchase of luxury items by the rich as conspicuous consumption was a problem. He banned professional guilds, except for the old ones, because many of them were subversive political factions. He abolished the exiting tax system and returned to the practice of allowing the cities to collect taxes directly, without needing Roman intermediaries. This ended tax farming, the exploitation of tax collecting by corrupt Roman tax collectors who used it to line their pockets.

On the political front he weakened the senate. He replaced the senators who had died in the civil war with his supporters and increased its size from 600 to 900 to fill it with more of his supporters. He ended the election of the officers of state and started appointing them instead. He abolished the practice of electing the officers of state who became his appointees, transforming them from being representatives of the people to being representatives of the dictator. This was a practice which continued by the emperors. He set the precedent, which the emperors followed, of requiring the senate to bestow various titles and honours on him. Finally, he had himself appointed dictator for one year (the normal term for this officer was six months), for ten years, and then for life. The dictator was an extraordinary officer of state who was usually appointed briefly to deal emergencies. He had more powers than ordinary officers. Caesar changed this into a permanent post for himself.