Urban Sketchers.org

I've just come across this blog which features sketches and often equally colorful stories behind the scenes by 100 invited artists in more than 30 countries around the world. Some are architects and illustrators, others are graphic designers, web developers, painters or educators, all sharing the same passion for drawing on location.
Urban Sketchers started online as a flickr group in 2007 and later became a nonprofit organization. Our mission as a nonprofit is to raise the artistic, storytelling and educational value of location drawing, promoting its practice and connecting people around the world who draw on location where they live and travel. We aim to show the world, one drawing at a time.

It's a great source of ideas and inspiration for sketching, as everyone has a unique and different style - take a look at the blog; you won't be disappointed.