The unfrozen great lakes often also has a silver mine or 2 on the island in the middle of the lake, which is a great source of coin that is unraidable as long as you have control of the lake, although if you don't have docks up or an age-up ship, it may not be worth going to and outposting, especially if you have more local mines, or are in fortress by the time local ones are gone and have a 2nd or 3rd TC up farther in the map by more mines.
EDIT: Wow, I'm nub, didn't even see the guy above me mention the mines.. :\
[This message has been edited by WookieBait (edited 07-04-2006 @ 03:30 PM).]