Is 117 HD Plugin free?

Yes. 117 HD Plugin is released under Open Source / Community and costs nothing. Neither it nor Old School RuneScape has any paid content.

What do I need to run 117 HD Plugin?

Old School RuneScape, which is itself free. 117 HD Plugin is an add-on and will not run standalone.

The PlayBound Launcher installs it in one click from Old School RuneScape's mods list. You can also install it by hand — full steps are on this page.

Roughly 30 MB, on top of the base Old School RuneScape installation.

117 HD Plugin — Free Old School RuneScape Mod · PlayBound