In my Northeast Minneapolis neighborhood, I've spotted a few interventions by and for bicyclists.
My favorite is the simplest: Under Broadway Avenue -- the neighborhood's busiest thoroughfare, a four-lane that takes cars over the Mississippi -- someone poured a concrete ramp, just the right width for bike tires, up the curb to take bikers through a sidewalk gate and beyond an often-locked chain-link fence across the road.
(Update: Contrary to what commenters at Reddit say, the design does incorporate a drain hole to let water pass beneath it, as these photos document. While the image above does show dried grass clumped into one side of this hole, it's not a fair argument that the ramp is the problem, as the unobstructed sewer grate down the street was even more clogged following the last big rainstorm. Can't blame guerrilla ramp-builders for that.)
On the other end, a sandbag makes a downramp. A simple, useful, anonymous and much appreciated urban modification.
More whimsical is a Sharpie addition to the lane marking on the nearby 18th Avenue bike path. A wear-your-helmet admonition, perhaps?
Farther afield, less bike-centric and more symbolic are British artist Pete Dungey's pothole gardens. While the plantings will quickly get destroyed, Dungey's site says the project's aim is to highlight "the problem of surface imperfections on Britain's roads," something we in the Twin Cities ought to consider highlighting.
Got tips on similar urban interventions in Minneapolis or elsewhere? Send them my way!