Apple released the OS X 10.10.3 update today, which includes a new Photos app that replaces iPhoto and Aperture, as well as a few other minor improvements.
The new Photos app is the big new feature here. It's similar to software like Picasa, where you can easily organise, auto-enhance and tweak photos. The new Photos app doesn't have the obnoxious menus and design of iPhoto. You can also create your own books and calendars from your photos pretty quickly.
It's hooked into your iCloud account, so everything on your phone can be synced up to the desktop app. If you've used Photos on iOS, you'll pretty much know your way around Photos in OS X already, though the desktop app has a lot more editing features.
Besides Photos, 10.10.3 also brings a set of diverse emoji and better support for Gmail's two-factor authentication.
Yosemite 10.3.3 (Free) [Mac App Store]