Oscilloscope
An oscilloscope is an invaluable tool for an electronics technician.
When trying to analyze the feedback from your design it can provide you with indespsible signal information.
If you do not have access to an oscilloscope you may use a PC soundcard based oscilloscope.
Although this may not justify every requirement, but it definitely serves the purpose for most basic needs.
Christian Zeitnitz has created this excellent tool in LabView.