Probably would be good to throw this stuff on wikipedia.
Each year Zero seems to tweak the frame, suspension, tires, brakes. The motor and battery pack were updated in 2012. A street-legal kit was available in 2011, and for 2012 the X was available only in street-legal form.
2008 has 1.7 kWh nominal 53V (molicel) pack, 20" front / 20" rear tires (16" street tire available), Mars brushed DC motor, optional Perm brushed DC motor ($900, +10% power), 140 lbs, $7450
2009 has 20" front / 17" rear tires, optional extreme package ($2000, Perm motor, Whites Brothers forks), 151 lbs, $7450,
2010 has 24" front / 17" rear tires, Hayes brakes (4 piston 9" rotors front/rear, still closer to DB brakes), optional extreme package, 161 lbs, $7500
2011 has 19" front / 16" rear tires, HB performance brakes (closer to motorcycle), 19" front / 16" rear, street-legal package ($500, lights / mirrors / plates), 185 lbs (+16 lbs for street kit), $7995
2012 has 2.6 kWh nominal 65V (EIG) pack, 19" front / 16" rear tires, Hayes brakes, Agni brushed DC motor, street equipment standard, 215 lbs, $9945