lsregister

Reset the Launch Services database.

Syntax
      lsregister: [Options] [path...]
                     [-apps domain[,domain]...]
                       [-libs domain[,domain]...]
                           [-all  domain[,domain]...]

Options
   -kill     Reset the Launch Services database before doing anything else
   -seed     If database isn't seeded, scan default locations for applications and libraries to register
   -lint     Print information about plist errors while registering bundles
   -convert  Register apps found in older LS database files
   -lazy n   Sleep for n seconds before registering/scanning
   -r        Recursive directory scan, do not recurse into packages or invisible directories
   -R        Recursive directory scan, descending into packages and invisible directories
   -f        force-update registration even if mod date is unchanged
   -u        unregister instead of register
   -v        Display progress information
   -dump     Display full database contents after registration
   -h        Display help

Paths are searched for applications to register with the Launch Service database.
Valid domains are "system", "local", "network" and "user". Domains can also be specified using only the first letter.

The Launch Service database controls the contents of the 'Open With' contextual menu.
lsregister is not on the system path so run it using the full path as below.

Examples

Run lsregister and display help

/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister

Reset the local, system and user domains:

/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -kill -r -domain local -domain system -domain user

This will also reset the first-launch security prompt: the first time you launch an application by opening a document, the system will ask 'are you sure'.

“Because the people who are crazy enough to think they can change the world, are the ones who do” - Apple Advert

Related:

Onyx utility - Includes a maintenance script to rebuild the Launch Services (use this to remove duplicate entries).
Article: TA24770 - Clear the 10.4 Finder "Open With" contextual menu (rename /Library/Preferences/com.apple.LaunchServices.plist).