I love watching the long coal trains pass though the railroad crossing near my home. Much of the graffiti is banal, but there have been some beautiful pieces. Of the simple text stuff, the Burma Shave homages are my favorites. Some of those are brilliant, with one phrase per car.
My town of Port Washington has some bland retaining walls that people are invited to decorate with chalk art. Upcoming free movies in the Veteran's Park Bandshell are sometimes advertised in chalk on a nearby wall by city employees who draw characters from the movies. It's great fun to see their original work embellished by others over the course of a week or so, and I especially love the little scribbles that sometimes show up about two feet off the ground, the clear handiwork of little ones in training to be the next Banksy.
The nearby city of Milwaukee, like Scylla's Toronto, commissions, both publicly and privately, glorious murals on otherwise unsightly surfaces. They have anti-graffiti efforts as well, but they try to get ahead of it by building civic pride, as Love describes.
As for SL graffiti, I did some in my early days here, by wearing alpha textured prims and striking static poses in front of things I wanted to tag. I often wore an alpha skin so I'd not appear in the image. Ultimately, it was easier for me to work outside of SL, in Photoshop. I have a Flickr gallery of other people's feed images I've modified over the years. I'd manipulate their snapshots, then post a link back to my Flickr version in their feed. My work makes little sense without seeing that I started with, but it's still there. Now I do it to images you people leave for me here.