Search versiontracker for "contextual menu". Here's a few that I found. You may find others listed as CM or CMM. Here's what I found first try.
Big Cat 1.0 _ - _contextual menu plugin for AppleScripts
BinJugglerCM 1.3.1 _ - _contextual menu for encoding/decoding MacBinary III
Contextual Menu Workshop 1.5 _ - _Framework for creating CM plugins
CopyPath 1.0.3 _ - _Contextual Menu addition copies path of current Finder selection
DesktopPictureCM 1.0 _ - _contextual menu sets the desktop picture
FCLPBReference 1.0 _ - _ProjectBuilder bundle: adds a contextual popup menu (OS X Server)
FileUtilsCM 1.3.1 _ - _contextual menu sets different file or folder attributes
FileXaminer 1.6 _ - _Get info tool w/batch mode & contextual menu
FruitMenu 3.1.1 _ - _haxie: customize & enhance the Apple & contextual menus
GrimRipperCM 1.3.2 _ - _contextual menu deletes resource fork of a file list
InternetLocationCM 1.4.4 _ - _contextual menu for Internet Location files
OpenMenu 1.1.0r5 _ - _Creates customizable contextual Finder menus
PicturePopCM 1.0 _ - _Contextual menu to view images
PicturePopProCM 1.0b7ex _ - _Contextual menu to view images
PopNav 1.0.1 _ - _ProjectBuilder bundle, adds contextual menus for navigating through source code
QuickAccessCM 1.5 _ - _contextual menu for frequently used folders/files
REALPlugJuggler 1.1 _ - _Contextual menu examines REALbasic plugins
SymbolicLinker 1.1v2 _ - _Contextual menu makes symbolic links inside Finder
In addition, several other commercial applications use CM plugins. One good example is Aladdin's Stuffit Deluxe