Tofurkey of course!
But really, try some of the more hearty foods and appetizers. Most of the vegi dishes that contain mushrooms (like a hearty stuffed mushroom) offer a more 'meaty' feel. Rice and beans make a perfect protein, quinoa is a nutty grain and is a whole protein as well. Most side dishes at the holiday table are vegetable based anyway, so a main dish with your vegetarian friends in mind would be appropriate and appreciated.
Most importantly, know that in most cases, people are thankful that you even tried to make sure they had something suitable to feast upon at the holiday table!