From Trimet Website:

Free Rides on New Year's Eve

Let TriMet be your gateway to a safe new year.

Go on, enjoy yourself—and leave the driving to us. Take a free ride on MAX, buses and the Portland Streetcar on New Year's Eve from 8 p.m. until the end of the service day. MAX Blue and Yellow Lines will operate until 3 a.m., so getting to the celebrations—and home again—is safe and easy.

Not sure yet where you'll celebrate? No problem; when it's time to go home, use TransitTracker by phone to get real-time arrival information for nearby stops.