What is 'positive impact', you ask. Well, I'm eager to hear your ideas but I'm thinking software that tackles the big challenges: energy usage, preventing armed conflicts, reducing poverty, STEM training, improving access to sustainable environments, implementing AI, et cetera.
I would argue Wikipedia continues to be a very important software project (albeit with an emphasis on the database on content than the wiki software itself), and Linux as this specific piece of software serves as a platform for many other applications and services.
What do you see?
Examples:
* burning fossil fuels externalizes the cost of dealing with climate change
* during the housing market crash of 2007, lots of risks were ultimately externalized to the state
* plastic waste ending up in the ocean means somebody[tm] externalized the cost of not properly disposing of / recycling their waste
... and so on, once you think in these terms, you find that pattern nearly everywhere.
If we had some kind of software solution to track externalized costs, that would be a huge step towards reducing it.
I know, this is very abstract, and I don't even know what a software solution for that would look like, but if somebody comes up with a really good of tracking that, it could have a huge impact on society in the long run.
Try to think of a society where nobody could quietly externalize a cost, and we had an effective way of tracking who externalizes how much, and go after the big offenders in a very data-driven way. There could even be general laws that make certain externalizations illegal, in a much broader way than current regulations do.