Maine is a really an odd state from the standpoint of the legality of online gambling because the codes would serve to make any form of gambling not specifically authorized by the state illegal:
"GAMBLING. A person engages in gambling if he stakes or risks something of value upon the outcome of a contest of chance or a future contingent event not under his control or influence, upon an agreement or understanding that he or someone else will receive something of value in the event of a certain outcome. Gambling does not include bona fide business transactions valid under the law of contracts, including but not limited to contracts for the purchase or sale at a future date of securities or commodities, and agreements to compensate for loss caused by the happening of chance, including but not limited to contracts of indemnity or guaranty and life, health or accident insurance."

Despite this code stating that it is illegal:
Unlike other violations of Maine law, Maine does not propose any sort of penalty for gambling. The way the laws of Maine are written, these kinds of codes are usually followed by some sort of prescribed penalty for violating the codes, but none can be found for the mere act of gambling.
They do have a separate code having to do with, “Advancing Gambling Activity,” but a reading of the code makes it evident pretty quickly that it only applies to operators. That part of the code also DOES go on to prescribe penalties for a violation thereof.
Additionally...
...they have a specific code related to, “Unlawful Gambling,” but again, the specifics of the code would describe someone acting as, “The house,” which is to say an operator, in the online context. They have a section for, “Aggravated Unlawful Gambling,” but that just has to do with the amounts in play within certain timeframes, and again, would apply only to operators or people acting as the house.
The Long and Short of it
As far as we can tell, the State of Maine does not penalize anybody for acting as a mere player. Even though the code describes gambling, there seems to be no code whatsoever that would apply to players.
For this reason...
...the majority of online casinos that offer their services in the United States, if not all of them, are willing to offer their services in the State of Maine. Here are some of them below: