Which is correct?
- He was pondering about the meaning of life.
- He was pondering the meaning of life.
The most common preposition is actually ponder over...
There's no difference in meaning, though some may think upon is dated and/or formal.
But in fact OP's second example (with no preposition at all) is by far the most common usage...
In most contexts where no preposition is used, one can directly replace ponder with think about, but think on, think over, and think upon don't normally work so well. This may lead some people to assume that ponder somehow "includes" the word about, so they find ponder about tautologous.
The second would be better.
As for the first, "ponder" is usually followed by either "upon" or "on," the latter being less formal and more common.
The meaning of "ponder (on)" is "think about." This is why the Preposition "about" is normally confused with it.