Steve Jackson has a small collection of useful genealogy utilities for working with MrSID image/raster files at his .Mac Homepage.
Unfortunately they are Classic/MacOS 8 & 9 only, but they include not only the browser plugins but a standalone MrSID viewer (that lets you export as TIFF files and that is much faster than going through the browser). Many of us run into these images, either through online maps we come across or through some sites publishing genealogy data in this format.
These are/were originally from LizardTech, however it appears they have abandoned the Mac platform. They can still be found there if you dig around.
Update: As of February 2005, mrSID support has been added to GraphicConverter.