You might have heard about Knitta, a "tag crew of knitters, bombing the inner city with vibrant, stitched works of art, wrapped around everything from beer bottles on easy nights to public monuments and utility poles on more ambitious outings".
Masquerade is doing the same thing in Stockholm, Sweden (which happens to be where I live). I haven't spotted any of these yarn tags yet, but will make sure to go knit graffiti hunting now. They've even put up a convenient Masquerade tourist map.
Seen above is "The Ring" from Munkbrokajen.
Link - via Craftzine