EasyZoom Giga Collage Maker
The EasyZoom Giga Collage Maker is a free software (not open source), which lets you create huge collages with unlimited size (in the gigapixel range) from multiple images. The resulting collages are in the “Smart Zoom Image” format (SZI), which is open source.
Collages can be uploaded, annotated, and shared/embedded through EasyZoom, which offers free space for up to 2 GB. An offline viewer will be available (currently under development).