MrSID, according to Wikipedia, is an acronym that stands for multiresolution seamless image database. It's a file format developed by a company called LizardTech for encoding georeferenced raster graphics, and it was developed primarily for GIS. MrSID images are popular and commonly used by GIS organizations.

Just recently we were asked if ArcGIS Explorer supported MrSID files, to which we replied - of course!

MrSID files are among the many raster file formats that you can open directly in ArcGIS Explorer. The Opening Content Help topic lists all the raster formats supported, along with all other data types. Beware, the list is very long. Here's the part listing supported raster file types (with MrSID highlighted):

To open a MrSID file, choose File > Open, then click Rasters.

Navigate to the folder where your MrSID file is located, and double-click to add it. Here's an example MrSID image shown drawn atop the USGS topo layer.