Redstone Circuitry - Resistors?

You can use a Redstone Comparator to produce a desired signal strength. When set to subtract mode, the signal being produced out the front will equal the back input minus the side input.

Therefore if you have a repeater directly behind the comparator (15 signal strength), and a lever with two pieces of dust going into the side (14 signal strength) the comparator will output 1 signal strength.

If the comparator has two side inputs, it will simply use the most powerful one.

Actually there is a circuit which is analogous to a capacitor using two comparators, some redstone dust, and some opaque blocks, but even they don't work exactly like real capacitors. I guess the closest analogous circuit I could think of to a resistor would be a comparator set for subtraction mode with another comparator feeding into it's side reading a set value from a chest. That way it can subtract a set value from an analog input. Obviously still not quite the same or really all that similar even, but I think it is about as close as you can get. You can even use an item frame instead of the chest to make it a variable resister, though I think you would actually need two if you wanted the full range of resistances (0-15), and you would need to subtract one from each before inputting them because item frames don't have a 0 output with comparators.

Of course this might not be terribly useful to you since it still won't work quite the same as a resistor in the real world.

Why not use command blocks? A simple /setblock command should do it.