Remember Settlers II? Well, Widelands has been going for a long time as a free and open source game inspired by it and they're progressing towards the big 1.0 release.
From a release standpoint, it's going to be absolutely huge. The changelog for the 1.0 Release Candidate out now is seriously long full of fixes, feature improvements and optimizations. It's difficult to pick out a few big features because there's just so much to look for. There's new maps, AI improvements, user interface upgrades, tons of gameplay improvements both big and small - seriously the changelog is mighty impressive.
It's been going for close to 20 years now which is pretty incredible. Widelands has grown into something quite special, that will easily appeal to fans of the classics.
See more on the official site.
If you want to download it easily to try out the RC, they have a Beta Flatpak package available.