What should I do with fuel drained from the sumps during preflighting?
The FAA issued an Aviation Safety Program publication called All About Fuel which talks about fuel preflight, including sumping fuel.  It contains the following:


  Preflight Action     As pilot in command, you have the responsibility to determine that your aircraft is properly serviced.
  Check your aircraft before each flight and be sure you have the
  correct type of fuel. This practice may save your life! Take the time
  to inspect your aircraft thoroughly. Good preflight procedures:
  
  
  Be sure all of the fuel and oil tank caps and covers are installed and secured properly after you visually check the fluid level.  
  Observe the color and odor of the fuel as you check the tank.  
  Draw a generous sample of fuel from each sump and screen drain into a transparent container. Check for the presence of water, dirt, rust
  or other contaminants. Unless you have the proper kind of fuel tester
  (e.g., GATS jar with intact fuel strainer), do not attempt to save the
  fuel drained from the sumps by pouring it back into the tank. Note: If
  you are not equipped to safely pour the drained fuel back into the
  tank, please be sure to dispose of it in accordance with EPA
  regulation.  
  Check that each fuel tank vent is clear of restrictions; i.e., dirt, ice, snow, bent or pinched tubes, etc.
  


Any FBO that sells fuel should have a way to dispose of it.  Some of them make it more obvious than others by putting sump barrels out on the ramp, but ask them what to do with it before you go out to the airplane and they should be able to help you out.  For environmental reasons, you definitely shouldn't dump it on the ground, because even if it does evaporate, it will come down somewhere.  At some airports you can even get in a lot of trouble (and have to pay a big fine) by dumping fuel on the ramp.  Don't take a chance!