Lowest denominator components to drive salvaged DC motor with 9v battery and Arduino?
Since you're talking about a toy motor and a 9V battery, I presume that we're talking low power. So:


PMV22EN small N-channel MOSFET  
1k\$\Omega\$ gate resistor (optional)  
1N4148 flyback diode  


That's two and a half parts.  

(the parts linked to are just examples; there are literally hundreds of suitable components)