Now that the FAA has added reregistration dates to their own site, I'm closing down this app and recommending you use them since they are 'official'. You can search for your N-number on this page and see when your registration expires. The advisory about the reregistration system is here. Hope the app helped, but now that better sources of info are available I'm recommending you take advantage of them instead.
The source code to this project will remain on GitHub, see the tech page for more info.N-Number lookup (see "Expiration Date" on the right hand side of the results)