SilverService is a Mac OS X application that allows you to use familiar unix shell commands from within any Services-aware application (i.e., almost any Cocoa application). Want to do a word count on part of a document? Select the text and invoke your wc-based service. Do a quick calculation in place? "Evaluate Python Expression" is your friend. Insert the current date? Look a name up in your Mutt alias file? Append a few lines to a file? If you can do it via the command line, you can do it in SilverService. The possibilities are endless.
Input can be provided via standard input or as part of the command; the output can be sent back to the application, or displayed in a separate window. Each user can define as many services as they want, added to the a submenu of the Services menu. Changes take effect immediately, so you can start to use the services the moment you create them.
For more details, see the ReadMe file included in the distribution.
SilverService is distributed under the GNU GPL. The current (but somewhat old) release version is 0.1:
Development versions are now hosted on GitHub.