Once upon a time, LizardTech, the owners of the Multiresolution Seamless Image Database (MRSID) compressed image technology often used with satellite and aerial imagery, had a free stand-alone viewer available on their website. Using this viewer, you could open full MRSID images, and export re-sampled subsets as TIFF images, GeoTiffs if the MRSID file was georeferenced or came with a world file. But LizardTech dropped the link for that stand-alone viewer download from their website some years ago, and now they only offer a browser plug-in that only allows viewing of MRSID images, and command-line utilities that convert the entire MRSID file into what could be an enormous TIFF image file.
But the Internet being what it is, it’s difficult to eliminate a file completely, and there are a couple of locations that still have the stand-alone MRSID viewer available for download. The Library Of Congress has many downloadable maps in MRSID format, and they offer a download link to both the viewer install program and the PDF manual (there’s also a viewer and manual available for Macintosh OS 9.x). The Spatial Analysis Laboratory of the University Of Arkansas at Monticello also has the install program, along with a downloadable PDF lesson on how to subset a georeference MRSID file and export it as a GeoTiff.
You can also use the free TatukGIS Viewer (Windows only) to open, view, and export MRSID files as well.