I always felt a little sorry for Fortress Forever [official site]. The folks behind the Source engine mod started in the long period when Valve had seemingly abandoned Team Fortress 2, with the goal of recreating Team Fortress Classic-y style class-based shooty shenanigans on a newer engine. Then Valve unexpectedly re-announced TF2, and Fortress Forever only beat it to release by one month before attention flocked to Valve's shiny new thing.
Fortress Forever is a pretty dang different game, though, especially after years of Valve bolting new things onto TF2. If you fancy some good old-fashioned 2forting, it's easier than ever, as the latest FF update brought it to Steam as a free standalone.
Fortress Forever is a fast, fast game, with the zippy classic-style movement full of tricks to exploit. TF2 has a few tricks, but old TF was a whole lot faster. The classes are quite different too, and objectives work differently (more flags!), so it's quite another game. The makers cite both QuakeWorld Team Fortress and Team Fortress Classic as inspirations, but it always felt way more TFC-y to me.
Fortress Forever is free for everyone to play from Steam. Here, now watch this old video full of men leaping around blowing each other to pieces: