[Ecky]

I'd like to throw out there that "hybridization" isn't something that can work as a bolt-on. The main advantages a Prius has over a Corolla, which runs a very similar ICE, is that the Prius is able to run the significantly more efficient Atkinson cycle, but which lowers torque at low RPM. The added electric torque makes up for this and allows for keeping the engine RPM down (often lower than the Corolla), and shutting the engine off at stop lights and in traffic creep helps significantly in city driving. Regenerative braking doesn't usually account for much.

If you just add an electric motor and battery to an existing drivetrain, you'll likely get single-digit percent improvements in economy. To get the real benefits a hybrid has:

1) You can emulate auto-stop by key-off at traffic lights and possibly engine-off coasting toward redlights and stop signs.

2) It's likely possible to run the Atkinson cycle in your Corolla. I'm not sure if Prius cams would fit, but you could likely have your cams reground or new cams made that convert to Atkinson cycle, plus a tune. In exchange for losing some power, you'll gain real, measurable economy. Probably not cost effective though.

3) Regearing can help in any vehicle, though you'll mostly see improvements on the highway. A taller top gear can easily be worth a double digit percent improvement.

If you're looking for quick, easy and cheap things that help in stop and go, look at cutting down on weight, especially at the wheels. If you can get a set of lightweight junkyard alloys, they might provide a measurable improvement. Also consider adding a kill switch.