I've been working on a program that takes a number of USGS DRGs and splits them up into seamless tiles. This image (140k jpg) (58k jpg) [27°22'30"N, 81°15'0"W] is one of my favourite examples so far of the differences between adjacent USGS topo maps, and should give you a taste of what I'm doing.

If it turns out that there are in fact NOT 1e6 open source programs that duplicate this functionality, I'll probably clean up and release the source.

Update

Map of area around my city

My overview-generator is working now. It recursively combines four higher-resolution images into one tile, scaling the higher resolution imagery by half. This is a piece of my newest map - click below for bigger versions.

(above map about 600 pixels wide, 52k)
(above map about 1200 pixels wide, 217k)