Are transponders required for hot-air balloons in the US when operating within a Mode C Veil?
I've gotten this question enough times to look up the regulations and answer my own question :)

14 CFR 91.215 lays it out quite nicely.  Particularly section B(3):


  (3) Notwithstanding paragraph (b)(2) of this section, any aircraft
  which was not originally certificated with an engine-driven electrical
  system or which has not subsequently been certified with such a system
  installed, balloon or glider may conduct operations in the airspace
  within 30 nautical miles of an airport listed in appendix D, section 1
  of this part provided such operations are conducted -
  
  (i) Outside any Class A, Class B, or Class C airspace area; and
  
  (ii) Below the altitude of the ceiling of a Class B or Class C
  airspace area designated for an airport or 10,000 feet MSL, whichever
  is lower; and


Where I see the balloons is definitely outside of the Class B airspace and below 10,000 feet MSL, so the answer is, no, they don't need a transponder.