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.

2.  Creative Class 

Richard Florida's consistently interesting ideas on why some cities flourish and others do not.

3.  Streetsblog 

A mash up of more than 230 local blogs covering cities, urbanism, transit and bicycling.  A wonderful sampler of cities.

4.  Newgeography 

Joel Kotkin has assembled a stable of writers who look at where cities have been and where they are going.

5.  Next American City 

Consistently good essays on place-making and cities.

6.  All About Cities 

Wendy Waters' unique perspective from her perch in Vancouver.

7.  Affordable Housing Institute: U.S. 

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.

8.  The Overhead Wire 

Expert in all things transit.

9.  The Anti-Planner 

Serious urbanists should make a habit of reading the serious anti-urbanists, and Randal O'Toole is their king.

10.  The Goodspeed Update 

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).