If you have a lot of guests who come over asking for the Wi-Fi password, then you know how much a pain it is to give that out while still keeping your network secure. DIYer NicoHood used a Raspberry Pi to power a QR-code system for easy guest logins.
Using a Raspberry Pi and a touch screen, the system generates a new passcode every day and when someone wants access to the network, they just need to scan a QR code. The system uses a second Wi-Fi network away from his home network, so guests get access to the internet, but nothing else. You'll find everything you need to make it for yourself over on GitHub.