A look around the majors and you'll find some rookies logging significant playing time, some for contenders. The following is a list of such players that have made a positive impact on their team with their bat.
Jason Heyward/OF/ATL- .290/.976, 26 r, 9 2b, 9 hr, 35 hr. The 20 year old phenom is proving many prognosticators correct in predicting him for the NL Rookie of the Year. As of now he's got himself in consideration for MVP too.
Brennan Boesch/OF/DET- .343/.966, 9 r, 17 xbh, 22 rbi. When Carlos Guillen went down with injury, Boesch came out of nowhere and has greatly exceeded expectations. Now that Guillen is almost ready to come back, the Tigers had to find a new position for him (2nd base) because Boesch is hitting so well and needs to remain in the lineup.
Austin Jackson/OF/DET- .335/.837, 31 r, 16 xbh, 13 rbi, 7 sb. Playing solid defense and providing a spark atop of Detroit's lineup. He may strike out a ton (53 k's), but his production outweighs that lone drawback.
David Freese/3B/STL- .316/.847, 19 r, 14 xbh, 28 rbi. Hasn't displayed much power yet, but has shown the ability to get a hit and play adequate defense at the hot corner for the perennial contending Cardinals.
Ike Davis/1B/NYM- .283/.866, 21 r, 13 xbh, 12 rbi. Some feel he may have been rushed to the bigs, but he's held his own thus far and has been an upgrade at 1st base from what was previously there (Murphy and Tatis).
Gaby Sanchez/1B/FLA- .277/.802, 22 r, 17 xbh, 21 rbi. On both of my fantasy teams and is a nice complementary player in Miami.
Honorable Mentions: Tyler Colvin, Ian Desmond, John Jaso, Sterlin Castro, and Reid Brignac.