I found this information regarding citizen's arrests in Maine. It's nice to know that if an individual witnesses someone committing any Class A, B, or C crime, he can do a citizen's arrest. Some Class D and E crimes qualify as well, such as obstructing government administration.
Here's a list of crimes by felony class (A-D). And here's Title 17-A, The Maine Criminal Code, where you can look up the list of Class D and E crimes listed in subsection 2 of Section 16 (Warrantless arrests by a private person) which an individual can be arrested by a private person for committing.