It's [_Euphorbia Maculata_][1] (or a closely related species), commonly known as _Prostrate Spurge_ or _Spotted Spurge_.  More images from Google [here][2].

It forms a tap root, so it's fairly easy to control by hand; my technique is to find all the branches radiating from one central stem, them grasp the stem just below the branching point and pull straight up.  I've also found that it responds well to Roundup, if you don't mind using weedkillers.  And just yesterday I used a flame weeder on some, which seems to be killing it -- I'll update in a few days with more details.


  [1]: http://en.wikipedia.org/wiki/Euphorbia_maculata
  [2]: http://www.google.com/search?q=prostrate%20spurge&tbm=isch