The venerable "Portable Air Tank", think compressor but without the compressor.  Simple and effective, this let's one take the air to the tire or other part of an aircraft requiring about 90 psi.

Portable tail stand / counter weight.  Remove the engine and all of a sudden the tail comes crashing to the floor, not a pretty sight.  Want to hold the nose wheel off the ground, weight it down.  This is a two for one tool.  Concrete supplies the weight, steel construction supplies the stand.

Jack stands, you need a pair for getting the aircraft wheels off the ground.  With the airframe jacked up off the ground, landing gear can be checked or repaired.  Often there is a requirement to suspend an aircraft for inspection purposes.

Maintenance stairs, these mainstays of working around aircraft give you the height to work on top of the wings and for refueling.  Many shapes and sizes are available, but the simpler the better.

Parts washer, also known as a solvent tank enables one to degrease aircraft components that are found top to bottom on your airplane.

Battery charger maintains the level of charge within the battery.  when a battery discharges to a certain point it is no longer useful at providing the cold cranking amps for which it was designed.  This usually happens at the most inconvenient  of times.  This one is even more unusual in that, it must charge a 24VDC battery. This need has been fulfilled- January 2010

Hydrometer, critical in monitoring the condition (specific gravity or density) of the electrolyte of a battery.  When the fluid in a battery will no longer respond to charging it is time to replace the battery.  Ideally before you are left in jungle with a battery to weak to crank the engine.