Write to NTFS-formated drives on Yosemite
Open Terminal.

If you have brewed osxfuse installed, you have to uninstall it, because unsigned kexts are banned now. Type:

brew cask uninstall osxfuse


On the other hand, if you don't have Homebrew at all, download it:

ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"


Updating Homebrew is recommended.

brew update


If you don't have Homebrew Cask installed, type:

brew install caskroom/cask/brew-cask


Install a binary osxfuse package from Homebrew Cask:

brew cask install osxfuse


Install ntfs-3g:

brew install ntfs-3g


You need to create a symlink for mount_ntfs, in order to make drives mount automatically:

sudo mv /sbin/mount_ntfs /sbin/mount_ntfs.original 
sudo ln -s /usr/local/sbin/mount_ntfs /sbin/mount_ntfs


If you are running OS X 10.11 and the commands above fail, try:

sudo nvram boot-args="rootless=0"


edit: This doesn't work for me in the final El Capitan (from Mac App Store). I had to disable SIP through recovery. Could someone confirm that this is indeed necessary?

... and reboot your machine. Then try symlinking mount_ntfs again.

Thanks to bjorgvino.