Threewave CTF for Deathmatch Classic (unreleased) - Grapple hook demonstration
Threewave Capture the Flag, a cancelled addon for Deathmatch Classic, has a grapple hook to make moving around easier. This video is a demonstration of some of the things you can do with it. I chose the map Strongbox for this since it has plenty of opportunities to use the grappling hook to your advantage.
The first person model for it is the one used in the Quake 2 version of Threewave CTF.
Threewave CTF itself is a port of the Threewave CTF mod for Quake to Deathmatch Classic that never came out, but was leaked in 2003 with the Half-Life 2 beta and other content from Valve.