I'm a big fan of these found street posters that have been archived on Ubuweb.com. All of them were discovered in New York City by Kenneth Goldsmith, the founding editor of Ubuweb, an online museum of the avant-garde. He's been adding on to the collection of strange posters since 1985.
In the project description he says
So, inspired by Jim Shaw's collection, Thrift Store Paintings, Adolf Wölfli's visionary scrawls, and outsider music, I began carrying a portable razor with me whilst out on casual strolls. What began as a hobby has remained an obsession and this obsession is brought to you in living color here on UbuWeb.
If you're as entertained by these street posters as I am, keep coming back to the page because new flyers are frequently added.
Here are a few of my favorites: