Whilst browsing XDA-Developers as you do of an evening, I discovered NaviComputer, which lets you download GPS maps on a desktop computer for offline viewing on a device.
It supports the following map sources
As well as simply displaying the maps, it has a display such as the one to the right with current altitude, time distance and speed as well as a graph of altitude. This is all well presented, and fairly responsive.
It needs a VGA/WVGA device (the developer says if there is enough request, a (W)QVGA one can be made, and your device still needs GPS, though you donâ€™t need an expensive data plan to be able to get Google maps anymore!