Timely hitting can do so much.
That said, the hits that end up really mattering are often the ones that are important only in hindsight--they aren't, necessarily, the walk-off hits, but ones that allowed the rally to keep going, ones that grabbed the lead or even tied the game, etc.
In no particular order I would include:
Alex Rodriguez's first swing home run against the Orioles in his first game back
Francisco Cervelli legging out a two-out infield hit that allowed Johnny Damon to hit a 3-run HR on Mother's Day
Brett Gardner's inside-the-park Home Run against the Twins
Francisco Cervelli's game-tying Home Run in Atlanta
And now:
Melky Cabrera's three run Home Run against the Red Sox.
What about you?
Which hits, walk-off hits or not, do you consider to be seminal moments in the season?