Apple's website for the Mac version of photos.app states that external editors and filter apps (extensions) will be able to be used. The iPad version achieves this, but I note they don't seem to be reflected to the photo stream.
Are there any apps that support this functionality yet on the Mac?
Best Answer
Looks like this is coming in El Capitan; it was announced at WWDC.
Reported on multiple news sites, but an example snippet from here: "...support for third-party editing extensions that will be available from the Mac App Store and accessible within Photos itself. It doesn’t sound as though that will enable external editing in Adobe Photoshop, say, but it will go a long way toward enhancing the editing capabilities of Photos."