Planting native evergreens like firs provides both cover and seed for small mammals like squirrels. They also provide food for seed-eating birds like finches, and insects and nest cavities for woodpeckers. If they pose no hazard, dead trees can be left standing to provide safe cavities as well. You can also install winter roosting boxes. Plans for roosting boxes are available from many books or can be purchased from your local bird shop.
Winter is also a great time to create a brush pile.
By collecting yard debris like branches, twigs and fallen leaves, you can create cover for birds and small mammals like rabbits, and at the same time offer a hibernation place for some species of turtles, salamanders and insects. The mourning cloak butterfly, for example, overwinters as an adult in woody debris and may use your brush pile as well. Many butterfly species and other insects, like ladybird beetles, will lay their eggs or overwinter, sometimes as pupae, in brush piles. Fallen leaves can also be used as mulch to protect plant roots from freezing temperatures, or composted and used as organic fertilizer next summer. Water can be scarce for wildlife in winter when natural sources are frozen.