Via this Google story I learned about MapKnitter, an open source
tool to upload your own aerial imagery and combine it into a
GeoTiff and TMS/OpenLayers map. MapKnitter is aimed at images
coming from balloons and kites. From their wiki: "MapKnitter
(MapKnitter.org) is a free and open source tool for combining
and positioning images (often from MapMill.org) in geographic
space into a composite image map. Known as “orthorectification”
or “georectification” to geographers, this step covers the
process of figuring out where images can be placed on an
existing map, and how they can be combined, or “stitched”
together. You are likely to have many images of overlapping or
identical areas, which is why MapMill or some type of sorting is
used to determine which source images to use from the original
set."