Currently the San Francisco font is the system font for all Apple devices (Mac, iPhone/iPad, Apple Watch, etc.) and Apple recommends developers use the sysetm font in their apps. However, this font won't show up in the font panel when selecting a font because it has a period in the beginning of the name. If you want to use it in the font panel, you can download it here and install yourself by dragging into /Library/Fonts/ or ~/Library/Fonts:
Unfortunately, the font is restricted by Apple license agreement for UI mockups and prototyping so the legality of doing this is murky. However, it is a great font for applications and actually has many great features that are exposed in the font panel 'Typography' pane. Apple should really make this more available for users.
Also, for developers, I'll give a plug for my library to make it easy to use the special SF font features in your app: