An app that inventories all your tools and checks them out when they are borrowed. SMS messages constantly nag your neighbor to return said tool(s) after a specific time interval. Sell with ads or go ad-free for $.99
I have wanted something like this that is voice controlled, but laser focused on remembering where things are stored. I don't think it would slow anything down to say things like "im putting a 1tb micro sd card in cabinet 4, shelf 2" while physically doing it. Then months later saying "im looking for an sd card" and being told out loud where to check. It could use entirely pre-set commands, be always listening but with 100% local processing and storage, and simplicity of the interface would be it's strength. I would run it on some kind of simple dedicated hardware as part of the space it inventories, rather than on a phone.