Well, there's
Navigator 4.4 Plus Europe. Written using QT, works both for Windows and Linux. The Linux version uses its own supplied qt shared library which produces really ugly un-antialiased fonts. I replaced it with the standard qt library from Fedora Core 6 and now the fonts look really nice. It wants to control a GPS device directly, but using gpspipe and a named pipe it can be tricked into using
gpsd.
I don't have a Car PC yet, but looks OK on my laptop.
[edit: sorry, no US maps]
Pim