OS X Yosemite introduced Extensions, which are listed in the Extensions panel in System Preferences. Currently, the list of "Actions" extensions on my OS X system contains only one item, "Markup".
How can I find more applications and utilities that can be installed as extensions? Is there a categorical list of extensions somewhere?
I tried searching the Mac App Store as well as Googling in general, but "extensions" is such a general term that it produces far too many unrelated results and makes it difficult to find true system extensions. Ideally, I'd like to find a list specifically about proper extensions (if such a list exists), or a better way to search for them (if there is one).