OS X El Capitan has introduced new system fonts, which are some variations of San Francisco typeface known as .SF NS Display and .SF NS Text. By default, they are "unlisted" in the Font Book, therefore unusable from applications like TextEdit, Pixelmator, etc. SFNSEnabler allows those usage in a limited way.
Simply download and install SFNSEnabler.otf. Now you should be able to choose the fonts from applications. If it's not visible in the application's usual dropdown menu, use the "Show Fonts (⌘T)" menu.
If you don't know how to install a font, see how. Ignore "Font Validation" by checking "SFNSEnabler.otf" and clicking "Install Checked."
.SF NS Text might not be visible, but you can still select most of them under .SF NS Display; G1, G2, G3 and the italics. Some application might behave clunky as the selected font name doesn't appear in the list properly.
Probably not. SFNSEnabler.otf is a simple blank otf file generated using FontForge, with only one metadata "Family Name" colliding with the system fonts. It does not use any of Apple's font codes nor glyphs.
Created by Dongsung Kim