The Sonic 1,2 and CD projects are cool since they are decomps of the remastered Android versions with proper widescreen plus modding support on 1 & 2 built in.
The Sonic Mania decomp allows you to choose your renderer (DX11 / DX12, and Vulkan), among bug fixes and mod support. Did you know the official release doesn’t let you use more than one controller in Competition Mode? The decomp lets you fix that. Plus mod support, of course. Most of the mods for Mania are cosmetic, but occasionally you get some cool stuff like more abilities or extending movesets to other characters.
Gamebanana.com is a good resource on all counts of classic Sonic modding
Yeah, the Doom 64 projects are a bit different in scope, so I figured they deserve separate listings. Anywho:
D64-RE is a decomp that you recompile yourself. There are some minor features like additional cheats, but it’s a pretty raw “here is all the code” project that you can mess with yourself.
Doom64 EX + is a fork of the original work by SVKaiser, called Doom 64 EX.
Doom 64 EX is a mish-mash of reverse engineering and source port conversion. Famously, Kaiser now works at NightDive Studios and brought his proprietary KEX engine with him.
EX+ basically rips out Kaiser’s KEX engine, while keeping the improvements of the modern commercial release like loads of bug fixes, performance increases, and aims to be faithful to the original game.
In addition, EX+ is able to be played on other platforms besides Windows, and can accept DeHacked64 patches. This allows for tweaks to values throughout the game (monster health, damage, weapons, etc), while bringing in support for a number of existing map packs.
Strangely, EX+ does not have controller support - at all. It is mandatory mouse/keyboard.