Now's a good time, Vertigo have released Family.Show, which is a sample application they were contracted by Microsoft to create to demonstrate the Windows Presentation Foundation (formally known as Avalon, aka funky XAML driven vector graphics thingy). So it's built with all the new fancy technologies, not only WPF but also ClickOnce.
It's basically a family tree/genealogy application. It also imports (and exports) the GEDCOM format which I'm told is used by other commercial applications, plus its free and open source.
You'll need Windows Vista, or (I think) Windows XP with the .NET Framework 3.0.