I think most of us here are broke *** more than environmentalists. But using less fuel at any cost is a noble cause. I mostly agree with what has been said, but I'll go threw your list as well:

1. increase intake air flow by replacing air filter.

I think this is bogus. The air filter companies disagree, but this is how i see it;
A mostly clogged filter is a restriction on you intake, like your throttle. If you never open it up all the way you will never notice the difference. I do recommend changing it when it looks really dirty, but you can stretch it further by taking a vacuum cleaner to it.

2. Computer programming. They suggest recalibrating vehicle's timing/shifting.

This has potential, but is likely going to be a expensive change, and may take lots of tweaking to really get results (which may not be great depending on the factory settings)

3. Spark Plugs. Replace spark plugs with mult-tipped plug that suppose to burn fuel more completely.

worthless gimmick (see Jay's post)

4. Exhaust. open exhaust to increase air flow with cat-back system and beter yet, high-flow cat and even headers.

This is much like the air filter. Why do you need increased air flow in the exhaust? Are you redlining you engine all the time? I can make a better argument for restricting exhaust;
With a restricted exhaust you will have higher pressure in the exhaust, therefor more exhaust will remain in the cylinders, and will take less air/fuel to make a proper combustion as the space is already partly filled with a gas that will not react to the combustion. This said, i wouldn't recommend doing this as it might not work at all, and may make things worse in other ways.

5. Synthetic Oils. replace oils with synthetic

A fine thing to do, but i wouldn't expect any gains, i didn't get any, but i do it anyway.

6. SVO. related to converting diesel to run on SVO.

N/A as you don't have a diesel. but a fun fact: animal fat gets you further than veggie

Yes, but we're looking at what's practical. IMHO if a mod won't pay for itself, its probably not worth doing, especially if its expensive. The most effective change you can make as far as economy is concerned is to modify your driving habits, not modify the vehicle. While it is noble to use less fuel at any cost, my main motivation is economic.

Spending little money is often the "greener" choice, as a lot of the money you spend, even on a new exhaust, goes to fuel burned to make it. It was once ore you know...

x2. I'm here because I can't afford to waste money that could be saved, and one area of my life where I'm willing to put in the effort to save is with my driving. I tend to answer questions from that point of view, often forgetting that some people have different motivations.

In that case:
Air filter - No. Do the math, replacing the air filter when it's not making a significant dent in your FE is bad for the environment. Consider the entire life cycle of an air filter: Materials are mined, refined, made into raw materials for manufacturing filters, then they are manufactured, then packaged (and you can consider the life cycle of the package), shipped, used, and then they get thrown into a landfill because they can't be recycled.

Environmentally, you should clean your air filter, and eventually if it manages to reduce FE (and the EPA's tests show otherwise), then replace it. Once it needs to be replaced, perhaps an oiled cotton gauze filter could be more environmentally friendly, because it is reusable and because cotton is probably more eco-friendly than paper.

Computer programming - This would be the big one. Find someone who can do completely custom tuning and is willing to work with you to adjust the tune until it's right for you. You won't find a tuner who has a lot of experience with fuel economy (since 100% of the tuning market is for performance), but if you find one who has a good fundamental understanding of how engines work, is patient, and open-minded...it could work.

Have him make torque converter clutch locking and DFCO very aggressive, let you open the throttle more without downshifting and with lower shift points, eliminate open-loop fuel enrichment under WOT, and experiment with lean burn cruising. Those are a few suggestions I can think of off the top of my head.

Spark Plugs, exhaust, synthetic oil - Not going to help FE or the environment. Well, synthetic oil will help FE slightly but I suspect that the energy put into it far outweighs the environmental affects of increased FE.

If you're burning waste oil, both are probably free and both should work fine in the same system. I wouldn't mind seeing WVO systems that also burn waste motor oil. Actually, I suspect that you can just dump it in without a problem (obviously, you'd have to filter it as well as you filter the WVO).

In most of the cases mentioned in this thread, there is a direct correlation between money saved and gas saved.

Air filter -- a clean air filter does help. Probably won't increase mileage unless you never change it.

Computer programming -- might help...mind what it may do to emissions, however.

Spark plugs -- been there, done that. Didn't help. Actually seemed to drop my mileage slightly...See my gas log.

Exhaust -- I've always heard that would help. Not enough money to try it...I'm waiting for someone with more disposable income than myself to try it and let us know...

Synthetic oil -- Noticed no gain in gas mileage, but oil consumption was reduced in my Geo which had over 150k miles when I began using synthetic. At the price Wal-Mart charges, I think it's worth it.

From what I've seen, the best results come from conservative driving, minding tire pressure, and some really good gains come from aero-mods.

if fuel consumption is the number one concern.....

buy a bicycle

too far for a bicycle.....

buy a 150-250cc scooter.

if it truly is all about your carbon footprint or consumption, then go all out.

well said Beef

also buy a used bicycle and or scooter as it take energy to make them, and there are already a lot out there

I actually considered a 250cc scooter or bike myself but I like the extra metal around me in the event of a crash (or deer).

This is correct. If the whole mod does not or hardly pay for itself than you're consuming more fuel than without the mod.
A general (economical) rule: the higher the price, the more energy or raw materials needed to produce/sell it. So cheap is the best if it's about saving gas.
A Prius is more expensive than my Civic just because it requires more energy and raw materials to produce the vehicle (including the batteries).

I agree with the no gain from synthetic oil idea. From what I know about oil (which isn't a whole lot) the intermolecular shearing forces of conventional oil are actually less than the synthetic, and therefore actually reduce friction. Theoretically going with low viscosity conventional oil would be the best for friction reduction. Having said that however, I change my oil with OEM recommended weight synthetic every 3000 miles. It's not worth the risk of experimenting with oil in my opinion.