You are free to tip and drivers are free to accept.
You can tip your driver at the end of your trip through the app. When prompted to provide a rating, first select a rating for your driver and then you will be given the option to add a tip as well.
You may also tip on a past trip through the app, on riders.uber.com, and from your emailed trip receipt.
To tip on a past trip from the app or riders.uber.com, select the trip from your trip history, and click 'Add a tip.'
From your trip emailed to you, click 'Add a tip.'
Finally, you may also see the option to tip your driver while you're on a trip. You'll be able to edit the amount of the tip at any point throughout the trip.
After a trip has ended, you have 30 days to add a tip in the app, on riders.uber.com, and from your emailed trip receipt. You can also give cash to your driver instead.
Note that on Split Fare trips, only the rider who originally requested the trip will be able to select the tip amount for a trip. If a tip is added by the original requester, it will not be split with the other passengers.
Uber takes zero service fees on tips.