We've talked about Nir Sofer before and his love of crafting nifty Windows tools and the developer shows no signs of slowing down. His latest application, NetworkOpenedFiles, lets you see which local files and folders are currently being used over your network.
The most obvious use of such a utility is security -- trojans, viruses and other nasties can stay resident in memory, opening up your network to allow outside connections.
Alternatively, if you're having trouble shutting down a program or deleting a file or folder, it's possible it's being accessed over the network. With NetworkOpenedFiles, you can easily spot the culprit programs and shut them down as necessary.
It's certainly not the sort of thing you'd use every day, but when you need it, it's absurdly handy.