If only the world was open source so many things could be improved, with the help of many, many people.
First, I would start off with writing a chemical filtering system to filter out the bad chemicals that are released from factories into the sky.This is most equivalent to a packet filter.
I would then start off with deploying that application into many cities and countries so that each part would be able to adjust it to their needs. A useful GUI would be good for this. This is also where cloud computing comes into major play, literally.
So after that, I would begin working on nature tools. Example, a daemon that sat in the jungles world-wide and reported statistics to us so that we would know when an animal is going to drift away.. thus maybe being able to save them before it's too late.
With that being said, this daemon would also report nasty spreading viruses, if at all possible.
I would then try to write an application that would scan the world for good places to dig up petrol. Lets face it, some places where oil companies decide to dig now is sometimes not good.
That doesn't seem like much when you first read it but if you let it sink in it'll become a lot.
I wrote this not to compare computers/software to humans only, I wrote it to compare what could really be done if people would help people to better the world.