Weight to be saved by removing a part or swapping to a lighter part:

Note: the '~' denotes the exact amount is not known, and that is a estimate/guess, most should be fairly accurate, though variation is possible depending on which aftermarket products are used.

This list was compiled by a former member of the MR2OC.com message board.

Trunk carpet and sound deadening material: 7.8 lbs
Spare tire and jack: ~45 lbs
Metal support that holds up the spare tire: ~2 lbs
Lightweight flywheel: ~10 lbs
Lightweight Clutch: ~5 lbs
Lightweight Racing Seats: ~20 lbs
Cruise Control: ~6 lbs
Lightweight battery: ~30 lbs
Downpipe: ~20 lbs
Aftermarket exhaust: ~30+lbs
Lightweight Wheels: ~15 lbs
Carbon Fiber hood: 24.4lbs
Carbon Fiber engine lid and sides: ~12 lbs
Carbon Fiber trunk lid: 9.8lbs
Carbon Fiber T-Tops: ~15 lbs
Carbon Fiber Sunroof: ~8 lbs
Spoiler Removal: 13.6 lbs
Power antenna motor: ~2 lbs
Lightweight steering wheel with no airbag: ~4 lbs
Rest of the airbag system: ~5 lbs
Floor mats: ~6 lbs
Carpet and sound deadening behind the seats on the firewall: 11.0 lbs
Front trunk plastic lining: ~1 lb
Remove trunk spring: 2.0 lbs
Power steering: ~30 lbs
Stereo and speakers: ~25 lbs
Air conditioning system: ~45 lbs
Replace Pop up Headlights with Flush Mount: ~15 lbs (per side - talk to Troy a.k.a trueleo)
Replace glass with lightweight lexan or Plexiglas: ~25 lbs
Lightweight Mirrors: ~15 lbs
Lightweight engine accessory brackets: ~15 lbs
Lightweight aftermarket brakes: ~35 lbs
Custom aluminum suspension pieces: ~10 lbs
Custom aluminum wheel hubs: ~20 lbs
Carbon Fiber Doors: ~40 lbs
Aftermarket engine management system: ~10 lbs
Harmonic balancer on rear crossmember: ~2 lbs
Manual locks and window conversion: ~8 lbs
Gut the interior: ~50 lbs
Windshield sprayers and reservoir: ~10 lbs if full
Metal knee plate behind driver's panel under dash ~ 2.2 lbs

Other Ideas MR2 Owners have shared
Heater element ~
Coolant lines to heater element ~
Coolant Bleeder Valve ~
Wiper arms and motors ~
Replace gas tank with fuel cell ~
Bumper reinforcements ~
Aluminum control arms ~
Custom tubed cross member ~
Remove heater controls (set up for derost only) ~
Remove EGR, charcoal canister and fuel pump resistors ~
JIC Magic FLT-A2s lighter than stock shocks ~
Alternator bracket weighs 11 lbs (find a cheaper alternative)
Aftermarket seats (the stockers weigh 36 lbs each)
Lightweight lugnuts ~
Remove side impact beams ~ 2-3 lbs
Front bumper reinforcement ~ 16 lbs

Some exact weight data:
Stock trunk lid: 14.8lbs
Stock '91 wing center section: 12.4lbs
Stock '91 wing side pieces: 1.2lbs each
Stock front hood: 31.4lbs
CF Trunk lid: 5lbs
Madpsi Competition Carbon Fiber Hood: 7.0lbs
Stock 91 Front Wheel+Tire: 35lbs
Stock 91 Rear Wheel+Tire: 40lbs
Stock 93 Front Wheel+Tire: 38lbs
Stock 93 Rear Wheel+Tire: 43lbs