I'm copy/pasting this from a previous response:
1.) there are
6 state forests in addition to Hodges Village Reservoir in Oxford, MA that are open for OHV
2.) for motocross and closed tracks, there are Crow Hill (Templeton, MA), Capeway Rovers (Middleborough, MA), Southwick, and the flat track up in Winchendon (which also has endurocross events)
all the sites are listed, and google map'd on thumpertalk's
Where2Ride section
3.) not posted on Thumpertalk, or anywhere else are Turkey Runs, Hare Scrambles, and Enduro runs through NETRA. Most of them require a street plate, but a few Turkey Runs per year are all on private lands, so you can run your yellow plated bike. They have a list of local clubs
here.
4.) there are a couple of illegal, but tolerated, places to ride that I don't know about, and private land (AND AS ALWAYS, DO NOT POST THESE LOCATIONS FOR EVERYONE TO READ)
5.) if putting a track system qualifies it as a snowmobile, it will open up a lot of legal riding areas so long as there's 4" of snow
6.) and lastly, I don't know if it's legal to ride on lakes and ponds (I called the town recreation department about a local one, and they said no), but the rules for most public ones say "(...in regards to ice) Snowmobiles, motorcycles, iceboats or any other winter recreational vehicles shall exercise reasonable caution and keep a reasonable distance from all persons...". If it were illegal, why would that rule be there?
for trails, the state forests that are closest to you are the best legal bet
and yes, since there are limited riding areas, a street plate definitely helps to connect trails and ride gravel roads (see theothersean's threads)