The Twittshirt website has been around for over 2 years and they feature 6 different color t-shirts and sizes XS-XXL. I love their design of the Twitter logo. I had searched for others and none came close to this design and with their choice of colors.
They take your Twitter username and produce a QR code for the shirt. 449 million people now have smartphones. Your shirt can be scanned with their smartphone and then they can follow you on Twitter instantly. They are 200,000 million people on Twitter with at least 100,000 million of them active every day. These Tweetshirts would be great at trade shows, social networking events and more.