Elvish Piper and Tradewind Rider on the battlefield.
Creatures you control can tap to produce at least ({G} magic symbol) .
You control at least one additional creature.
Tradewind Rider and Elvish Piper do not have summoning sickness.
Steps
Activate all mana-producing creatures you control by tapping them, adding at least ({G} magic symbol) .
Activate Elvish Piper by paying ({G} magic symbol) and tapping it, putting Village Bell-Ringer from your hand onto the battlefield.
Village Bell-Ringer enters the battlefield, untapping all creatures you control.
Activate Tradewind Rider by tapping it, Village Bell-Ringer and another creature you control, returning Village Bell-Ringer from the battlefield to your hand.
Repeat.
If you can produce at least one additional mana in step 1, this combo produces infinite mana.