There are several. The Appalachian Trail can be access in several spots like the Allagash, Bingham, Jackman, and other spots. There is a wonderful trail that runs along the Messalonskee Stream in Oakland. There is one of my favorites called Hobbit Land in Hallowell. Those are three that I frequent the most.
The Appalachian trail is the most beautiful when accessed from the Allagash or Jackman area in Northern Maine. Oakland and Hallowell are both in Centeral Maine.
Hope that helps.