FWTools - MrSID Plugin
Lots of folks are interested in MrSID support for GDAL and FWTools. 
I would love to distribute such support as part of the core FWTools
but the MrSID DSDK has a variety of license restrictions which it is
hard for me to be sure I could comply with to the full legal letter of
the law.  So, to play it safe I instead offer it as a seperate
plugin and require users to download the DSDK from LizardTech, thereby
agreeing to their license agreement directly.
I applogise for the inconvenience of this.
Win32 - Download and Install DSDK From Lizardtech
-  When installing FWTools, select the MrSID Plugin option.  This
is available in FWTools 1.0.0 and later. 
 -  Go to Lizardtech Developer web site: developer.lizardtech.com
 -  Register as a user and login. 
 -  Select "Download SDK's". 
 -  Review and agree to license agreement. 
 -  Download "GeoExpress SDK for Windows - vc7".  I am using
Geo_DSDK-5.0.6.955.win32.zip, so hopefully you will get the same
version. (32MB!)
 -  Unpack, and copy the Geo_DSDK-\lib\Release_md\lti_dsdk_dll.dll to
FWTools-\bin.
-  Test in FWTools Shell command window with the command "gdalinfo --format mrsid".  You should get some brief info on the format if the plugin is working.
  
Linux
I plan to offer the plugin for linux as well shortly, but it is not yet 
available.