What is the converted mana cost (CMC) of land cards? Is it zero?
For example, if I have lands in the graveyard, would Sun Titan be able to bring them to the battlefield?
This is also noted in the rulings below the card in Gatherer:
If a card in your graveyard has no mana symbols in its upper right corner (because it’s a land card, for example), its converted mana cost is 0.
†: with the exception of transformed lands, as noted in @NeilMeyer's answer. This situation doesn't happen in the graveyard, though.
Certain double-faced cards from Ixalan block become lands when they are transformed. These lands have the CMC of their front face which is often not equal to 0.
711.4b While a double-faced permanent’s back face is up, it has only the characteristics of its back face. However, its converted mana cost is calculated using the mana cost of its front face. If a permanent is copying the back face of a double-faced card (even if the card representing that copy is itself a double-faced card), the converted mana cost of that permanent is 0.
202.3a: The converted mana cost of an object with no mana cost is 0, unless that object is the back face of a double-faced permanent or is a melded permanent.
So yes, Sun Titan's triggered ability could bring a land to the battlefield.