If you're thinking about learning how to test electronic components with a multimeter and perform electronic repair, from the convenience of your own home, and you're looking to spend as little as possible to get started, then look here.
Transform your computer into an oscilloscope with this tool. WARNING: Don't use your soundcard directly to send and receive tones. Use a linedriver to protect the inputs and outputs of your soundcard.
Measure signal/noise, distortion and lots of other good things with RightMark Audio Analyzer: