Can you sacrifice a land to pay a cost during your main phase if you have already played a land this turn?

There are two completely different actions that involve lands: playing a land and sacrificing a land. Playing a land (CR 305.2) is a special action governed by the rule that limits you to one land per turn (CR 305.2b). Sacrificing a land as part of a cost is an entirely different game action covered by CR 701.17.

Sacrificing a permanent means you move it from the battlefield to its owner's graveyard as part of paying a cost or meeting a requirement (CR 701.17a). This action is never restricted by the one-land-per-turn rule, because you are not playing the land — you are simply using it as a resource to pay for something.

So if you've already played your one land for the turn, you can still sacrifice any number of lands you control to pay costs — for spells, activated abilities, or any other effect that demands it — as many times as you have the mana/resources to do so.

Concrete example: You play a Forest on your turn (using your land drop). Later in your main phase, you activate Zuran Orb's ability, which reads 'Sacrifice a land: You gain 2 life.' You may legally sacrifice another Forest you control to pay this cost, gaining 2 life, even though you already played a land this turn.

Remember: the one-land-per-turn rule (CR 305.2b) applies only to the act of playing a land from your hand as a special action. Any other game instruction that says 'sacrifice a land' is completely unaffected by that restriction (CR 701.17).