I made a tinker-box, as recommended by Bob Pease in Troubleshooting Analog Circuits. It's simply a box of banana jacks connected to various resistors and capacitors. There is one rotary switch connected to resistors of values 10-66kOhms. There is a second rotary switch connected to capacitors of values 10p-1uF. Finally there is also a 100k potentiometer and a ferrite bead. Not much else to say about it other than that it's of great use when experimenting with op-amps. I used it when i built last posts EMC-meter.