Is static negative resistance practically available?
It's easy to simulate a negative resistor with one side grounded. See also this Wiki entry. For example: 







simulate this circuit &ndash; Schematic created using CircuitLab

Here the circuit to the right of the capacitor has an impedance of -R1(R3/R2). 

I've demonstrated in this case to cancel out the resistance of the inductor R4. Here you can see the results with R1 = 0 and R1 = 100 ohms:

R1 = 0



R1 = 100 ohms   



For reasons that should be obvious, it's a bad idea to completely cancel out the positive resistance in most cases. Some DC motor controls cancel out much of the armature resistance to improve speed regulation under varying loads. 

You can also look at GICs (Generalized impedance converters), which are related.  

This is an interesting parlour trick, however usually there are much better ways of doing filtering or whatever it is that has to be done.