HunCraft Genocide is maybe the most detailed unofficial expansion pack for StarCraft: Brood War. It has 3 new units, new heroes and special buildings in the campaign, 3 new campaign episodes (27 missions and a hidden mission), more than 40 new multiplayer maps and some small changes. Though the most StarCraft expansion packs can be used in use map settings, this game has own exe file, menus and new map file format (hcx). It does not overwrite anything to your original StarCraft installation, but needed to be installed in the same directory. There is a bugfix patch for the Hungarian edition which make some balances in the game. Single player can be described with mass battles and a bit more difficult than original StarCraft. The story continues after Brood War, when admiral DuGalle has committed suicide and Kerrigan has regained control of the Zerg forces.
Quality over quantity mod. New units fill in gaps of each race without being OP. Not to much new content, no change in mechanics - this is absolutely fantastic!
If only it was also available in English, as well.
Phenomenal campaign with relatively good plot and great voice acting. (Spoilers below)
(SPOILERS - Too many characters die too quickly IMO, but I can't argue that this is a good or bad thing - plot-wise, everything still works pretty well.)