Please don't restrict this to web developers. This could be useful for lots of other fields:

    - Academics: This data just got published, let's analyze it!
    - Native app programmers: Open source application X lacks 
      feature Y, let's add it!
    - Embedded system developers: The arduino/mbed/BeagleBoard lacks
      a driver for this system, let's write it!
    - Physical constructs: This seniors neighborhood in Grand Rapids
      lacks wheelchair ramps, let's build some!

Some would obviously be location dependent, and you'd be much more dependent on tags to filter content (which is OK, as long as you have a good system to manage tags), but this could be much bigger than it currently is.

With sufficient size, you could probably get some good advertising revenue - users are willing to tell you precisely what they love to do, and I don't think they'd mind a few ads on these topics. I'd definitely go for the ad-supported model for this one.