BBEdit is my favourite text editor and I’ve been a user since way before Mac OSX. The latest update BBEdit 11.6 brings a number of bug fixes and updates but also introduces a new demo mode. For 30 days the full feature set is available for free after which time BBEdit will remain permanently functional with a revised feature set that includes its powerful text editing capabilities but not its web authoring tools or other exclusive features. These features may be re-enabled at any time with a purchased license.
BBEdit 11.6 requires Mac OS X 10.9.5 or later, and is compatible with OS X 10.11 "El Capitan".
BBEdit can be obtained from the online store.
There is also a listing of markdown editors here