The experimenter's resistor pseudo-decade box. It's not a real decade but two precision "ten turn" pots of different values (http://ebay.to/2zpSskz) and a hard coded resistor at the end in series gives a lot of flexibility for the experimenter on a budget. I use a 1k Ohm pot, a 50k Ohm pot, and a 1M ohm hard coded resistor for lots of options. You can bypass or simply change the last 1M resistor via banana jacks if needed.

Knob options are plenty, but I used and love this openSCAD option (kudos): https://www.thingiverse.com/thing:54024

Make it your own!