Way back in 2001, London.pm organised a series of tshirts. As Dave Cross explained:
They all have a large "Programming Republic of London" logo on the back and one of a series of small "fictional" PM group logos on the front. I'm currently wearing my Sunnydale.pm shirt, but we also have Rivendell.pm, Tatooine.pm, ZZ9PluralZAlpha.pm and Anhk-Morpork.pm (the last is for those strange people who can't see that Terry Pratchett is an unfunny, fifth-rate Douglas Adams wannabe).
Also, see the images as Simon Wistow explains:
Yes, the rumours are true. After epic battles with every t-shirt printer in london the official Winter Season London.pm t-shirts are finally ready.
Embarassingly we left out our favourite fictional Perl Monger group. However, it was, how shall I say, quite a lot of organising and quite tiring to ship tshirts around London and the world. Luckily, now we live in the future and I can outsource all that, so behold the official London.pm Gallifrey.pm tshirt.
It's being sold at cost and you can wear one at all fashionable geek gatherings. Enjoy!