Annie Mole's, webmaster of Going Underground, daily web log (blog).
If you like this you'll LURVE One Stop Short of Barking, THE fun and informative BOOK about travelling on the London Underground.
Started on the 25th April they wrote "At 13:54 Stickers on the Central Line took its first steps. After months (well hours) of planning we were finally ready. Armed with stickers we boarded the 13:50 from Epping to West Ruislip and unleashed phase 1 onto an unsuspecting public."
The stickers are pretty inventive and each one comes with a commentary on the blog. I love the explanation for Lille: "It is a little known fact that between Woodford and Buckhurst Hill there is a lightly used tunnel which provides London Underground commuters direct access to the Paris Metro. Providing you’re not in a hurry, you can just sit back (if you can find a seat), relax and simply show your passport when the border guards pass through the train."
As well as the blog, there's also a Facebook Group where you can record your spots and send in ideas for stickers.
Thanks to the many people who let me know about this and hat-tip to @Londonist on Twitter where I first saw it mentioned.
"The magic, mystery & sometimes maddening shortcomings of London's Tube are documented with love, enthusiasm & sometimes despair by its unofficial social historian." The Guardian