On the 15th of April 2010, you can get a free coffee from your local Starbucks!
Here's what they are saying:
Join the movement. On April 15th, bring a reusable travel mug into your local Starbucks and get a free brewed coffee.
Most of us have one of those travel mugs lying round, so this should be an easy freebie to claim.
UK is in the list of applicable countries at the bottom of the page :) but it does say "Offer varies by country, ask your barista for details."