Here are some of my favorite Contextual Menu items/applications:
FinderPop gives you the ability to have quickly-accessible links to applications or folders at the top of your contextual menu. So, for instance, if you want to open a JPEG file with Photoshop instead of Preview (often the OS's default), you can put a link to Photoshop in your FinderPop contextual menu list and then right-clicking on the file in question will give you a quick list of applications at the top-most menu. This is faster than having to wait for the "Open With" application list to generate, which is useful if you have a lot of applications installed on your system.
OnMyCommand lets you execute a string of selected text in the Terminal.
SymbolicLinker is a contextual menu item that lets you build symlink to an item in the Finder. I've found this very useful in building lab and classroom images where my workstations have much of their shared data as symlinks to other locations in the filesystem. For instance, Microsoft Office 2004 likes to install 80MB of fonts for each user but I don't want dozens of user profiles each with 80MB of fonts on my workstations, so I've used symlinks to "trick" the program into thinking that each profile has the fonts installed but the actual fonts are located in a shared folder space. But because the symlink exists (but points to a different location), the software just follows the link. I've used the same trick for the excessive support files installed by the Macromedia suite.
FileCutter brings a much-desired Windows feature to Mac OS X--cut and paste. I know a lot of Windows guys who are pissed that Mac OS X doesn't use the same "cut and paste" paradigm they are used to from years of Windows use. One of them actually jumped in the air and clicked his heels together when I showed him FileCutter. (Okay, he didn't click his heels together, but he jumped out of his chair).
There are more CM plug-ins out there. Some applications install their own, like Toast or StickyBrain. Which ones do you use and why? Are there any "must-haves" that I've missed?