This has been around for sometime now, however I can't find anything on Ozbargain and I know some people aren't aware that it applies with Myki.
Free travel on Melbourne's electrified train network for trips finishing before 7.00am (weekdays). However technically it is before 7.15am exactly. I believe they have done this with Myki to allow for trains that are late. If your train arrives after this you will be charged.
You need to have a Myki and positive balance. You need to touch on and off and you wont be deducted anything.
If you only train into and from the city then this essentially halves the cost of your travel as you only pay for a 2 hour fare.