Recently, GOG started selling Diablo. Soon after, it added Hellfire to the deal. I was aware that Diablo had that expansion, but it wasn't great, and was largely forgotten. I'm not sure why GOG has bundled it, but I'm cool with it being there, because I respect complete collections. Although I've played Diablo, I don't have nor plan on playing Diablo Hellfire.
That reminded me that I had read (probably from some wiki) that StarCraft had two expansions (other than Brood War) that were similarly swept under the rug. I had only vestigial knowledge of them, and had never played nor seen a copy of either in person. A search uncovered ISOs on the Internet Archive, so I installed them on Twentieth Century, and got playing.
Both are map-only expansions that are accessed through the custom game menu. They don't add units, map types, or music, and gameplay is functionally identical to StarCraft. It's just like someone threw these together in the map editor. That might sound like a disappointment, but I'm not going to ding these for it. For the record, I played these with Brood War, not the base game. While campaigns in both expansions are fully voice acted, the quality varies. Some voice actors have a much better sound quality than others.
Insurrection revolves around a Terran settled world. Like the base game, there are three campaigns for the three species, and it starts off with Terran local law enforcement encountering Zerg. Like the title suggests, all three factions eventually have some kind of rebellion going on. Some of the maps were recycled, but you're always starting from a different corner. That's OK as long as it's done well; I've had that happen before. For example, there's a large battle at the end of the Protoss campaign, and the first mission of the Zerg campaign is to destroy everyone at the same spot!
Retribution revolves around all three species going after a stone, which is the entire expansion's MacGuffin. It's not the voice acting that bothers me; it's the fact that only one character that talks to you in each campaign. It wouldn't surprise me if it's the same guy for all three! Some map's design is lazy. One was entirely flat, except for two opposite corners (your base on one and your objective on the other).
I'm sure that none of these are official canon. If you want something other than the usual free for all or melee map action, you won't be disappointed, but don't get your hopes up for a good story or performance. But hey, they are free, and if cringy sci-fi is your thing, awesome!