Did you know Maine (the only one-syllable state!) used to be part of Massachusetts? No, really. It became its own state in 1820, though state archive records go back even earlier.
The Maine Archives Interactive site is a master index to (mostly offline) archived materials such as photos, maps, court cases and the usual arcana of state archives. There's a hodge-podge of items and records here:
You can download databases of the archives indexes. Materials include indexes of photos, legislative sessions, land grants, soldier rolls, town records and so on. Remember, these are indexes rather than actual archived materials.
The University of Maine Library System has additional online resources, presented by category:
Lastly, there's a nice list of Maine historical resources, both online and offline.