"Why do some plants grow Witch's Brooms?", a teacher once asked. Before anyone could respond, he answered, "So that we can identify them more easily, of course."
Though said in jest, it's nonetheless true that many odd plant growths are species specific and can serve as shortcuts to identification. Here in coastal southern Maine, growths of crowded woody twigs, called witch's brooms, allow for quick recognition of Highbush Blueberry (Vaccinium corymbosum). While similar abnormal growths do occur on other woody species, locally this host seems the most common. Witch's brooms are caused by infections brought on by various organisms. In the case of Highbush Blueberry, the brooms result from a rust fungus known as Pucciniastrum goeppertianum.¹ I don't recall seeing these brooms when I lived in Plainville, MA; likely because Balsam Fir, the alternative host of the fungus, is uncommon in that part of the state.
Another odd growth on Highbush Blueberry that I did see in Plainville is the blueberry kidney gall.² This gall forms around the eggs of a small wasp (Hemadas nubilipennis) and serves as a larval shelter. Galls from previous seasons will often show multiple exit holes -- evidence that the insects have emerged. I've yet to notice any of these in southern Maine; perhaps the wasp prefers a milder climate. (Feb 2015 update: I've since found plenty in the area.)
Absent the above clues, look for the plump pink/red buds on winter twigs and brown bark on older growth that shreds into thin strips. For more help recognizing this widespread fruit-bearing species, check out the images below. (To view them in full-size, click here.)
¹ Witches' Broom, University of Minnesota Extension.
² Stokes Guide to Nature in Winter by Donald Stokes (1979) discusses these and other commonly encountered galls.