With C&C4 leaving some big huge gaps in Tiberium-town's long-running lore, it seems only right that the series' fans would fill in the gaps. Better to do it with new game-stuff than with long, rambling fanfic, too. C&C3 mod the Forgotten is a three years in the making tale of a rag-tag band of space crystal-mutated lost souls, and the part they played in the Third Tiberium War. Introduced in Tiberian Sun: Firestorm, they basically got dropped from the story from thereon in. Forgotten, if you will.
An 863Mb endeavour, offering new HD cutscenes as well as a slew of new units, levels and icky mutation graphics, it's a meaty old project. There's a release trailer below, and you can download it here.
And here's one from last year, showing off the cutscenes they're on about, plus in-game unit death-bang-bang-splode fun.
It's made by a bunch of chaps known as CNClabs, and they've provided a ton of detail on units, buildings and backstory right over here.