I've excluded Austin blogs and blogs that focus on just one city. These are in no particular order.
1. Ryan Avent
Elegant writing on urban economics and transit.
Richard Florida's consistently interesting ideas on why some cities flourish and others do not.
A mash up of more than 230 local blogs covering cities, urbanism, transit and bicycling. A wonderful sampler of cities.
Joel Kotkin has assembled a stable of writers who look at where cities have been and where they are going.
Consistently good essays on place-making and cities.
Wendy Waters' unique perspective from her perch in Vancouver.
Don't let the stuffy title throw you. This quirky blog covers the ecology of India's slums one day and the "ultimate future city" the next, along with knowledgeable posts about the obstacles to affordable housing in American cities.
Expert in all things transit.
Serious urbanists should make a habit of reading the serious anti-urbanists, and Randal O'Toole is their king.
A planner's thoughtful perspective on the obstacles confronting livable cities.
Honorable mentions: Market Urbanism, Emergent Urbanism, Environmental and Urban Economics (wonkish) and Matthew Yglesias (only a small percentage of his posts deal with urbanism, but those that do are usually very good).