Air conditioning unit sitting on the ground
Code requires outdoor condensers be installed on a reasonably level surface raised at least 3 inches above ground level.  If water pools in the lower part of the unit during cold weather and then freezes, the condenser coils can be damaged by ice expansion and that will often end the life of the unit, although the outdoor condenser can sometimes be replaced.

As Tester101 said, there are prefab pads made for this and if you can raise your unit without damaging any lines that are connected then you may be able to do this without an A/C technician.  Great care must be taken not to kink or break the refrigerant lines if you attempt this.  Otherwise, you will need an A/C contractor to recover the old refrigerant and disconnect and temporarily cap the lines; and then after you install an acceptable pad the A/C contractor will need to install a new refrigerant dryer, reconnect the lines, purge the lines with nitrogen, evacuate the system to a deep vacuum, and correctly recharge the system with new refrigerant.  If everything goes well this is around a half-day of work (not including installation of a new pad).

By Request:

2009 Uniform Mechanical Code 1106.2 "Supports and Anchorage" last paragraph:


  A compressor or portion of a condensing unit supported from the ground
  shall rest on a concrete or other approved base extending not less
  than three (3) inches (76mm) above the adjoining ground level.


Admittedly, this language does not use the phrase "reasonably level" and it is not the most current version.