The Tampa Bay Rays have experimented this season with having one of their relievers pitch the first (and sometimes the second) inning, before switching to a pitcher more accustomed to working for many innings. People have taken to calling these repurposed reliever "openers" - the opposite of a "closer".
Teams score in the first inning more often than any subsequent inning, since a team is guaranteed (assuming its manager fills out a fairly traditional lineup card) to send its best hitters to the plate. The "opener" strategy allows the reliever to face a lineup's heavy hitters right away, giving the next guy (probably a starter) an advantage on his third time through the lineup.