Shoestring
I’m told that this sort of thing is popular in the USA. Popular culture has it that it’s gang or drug related, although snopes seems to say that’s unlikely. I suspect the kids round here have bought into the whole “it’s a gang thing” story though and think it makes them look hard.
Personally, looking at the shoes, I couldn’t help thinking “What a waste! Those have got loads of wear left in them” – but I’m a stingy old fart.

We have a shoe tree in one of the local parks – http://www.flickr.com/photos/duncanh/370789951/
Of course, the council don’t like it and remove them once or twice a year but they always start appearing again.
It got a mention in a Guardian guide to the city so is now an official tourist attraction :-)
There used to be a Shoe Tree down on Collage Green a few years ago – for all I know its still down there. Bristol CC keep on taking the shoes down but then they use to just gradually reappear again! Looked very cool, especially as it was in Spring before all the leaves- was on one of the trees near the Cathederal.
Mary, it’s still there:
http://www.tlmb.net/galleries/SpringWalks/pages/page_18.html