The Portland Trails website offers maps and refined searches to tailor to just the hike or walk you are seeking. The link above provides information as to which trails are snowshoe and/or cross country ski friendly.
We recommend the Gilsland Farm - Maine Audubon. It's only a 10 minute drive from downtown Portland; rent snowshoes for $5! You can watch the incoming tide and seabirds from a bluff overlooking the Presumpscot River estuary.