Windows only: RouterPassView is a freeware application that taps into a router's configuration file, allowing you to recover important data like router login information, wireless network keys, and more in the event any of that important information goes missing.
Once you load the configuration file into the program, it displays your (supported) router's passwords, wireless keys, and login information. It can also decrypt ISP usernames and passwords. It could be really handy if, say, you're troubleshooting a less tech-savvy friend's router or if you happened to lose that information yourself.
RouterPassView supports a limited number of models, so you'll need to check the web site to see if your router will work with the program. It includes some popular router brands like Linksys, D-Link and NETGEAR. Support for additional routers is planned for the future. RouterPassView is a free download for Windows only.