Not quiet sure I understand the question posed.
How about contributing to some open source software? The question seems to be focused only on personal usefulness, I'm sure there's stuff out there that people already find useful that you could perhaps help improve on.
You could also do a lot of stuff with a personal server if you were so inclined, you wouldn't be writing code from scratch but your understanding of code would help you in understanding some of the nitty gritty details in setting such a thing up.
What skills to learn, what tools to get