Some time ago, Pete (an occasional commenter here and an Applescript Ninja) pointed out a new application for writing your own Service menu items. At the time, it was called Concierge, but due to a clash with an existing application, they have renamed it to Bellhop. The idea is that Bellhop provides you with a scripting interface to the Service menu (currently you can use Applescript or Ruby), so that you can write scripts in your language of choice to take data (usually text) from one Cocoa application, do something to it, and pass it to another Cocoa application. I think it is possible to integrate services into Carbon application, but very few developers do it.
It's quite nicely set up, with the ability to group your scripts together in submenus, and also rebuild the service menu so that you can see the changes without logging out. It's certainly worth a look if you know either Applescript or Ruby, and would like to integrate your applications better, or just add a new feature to a Cocoa application. However, it seems that they will eventually be charging for the product (at least for some of the functionality), and there is no indication yet as to what that price will be. So don't get too hooked before you find out how much you'll have to fork out to feed your habit.