Red Hand of Doom
Red Hand of Doom is Rich Baker's module for Dungeons & Dragons 3.5 edition. The players are attacked by a Hobgoblin warband while walking down the road. A quest given by the nearby town to clear the nearby Hobgoblin base leads the players into conflict with an army of Tiamat worshiping Hobgoblins and their monstrous allies.
It's pretty much the only Wizards published 3e module that gained widespread popularity aside from Sunless Citadel. Popular enough it often appears in overall top ten positions across all first party D&D modules regardless of edition.
What makes it so interesting? Firstly the module is somewhat sandboxy, tasking the players to do various things to slow down the horde (which matter) and not as railroaded as most published modules (Be warned: The Hobgoblins do not move at the speed of plot). Secondly the scale is quite grand, few modules convincingly have the party face an entire army, and includes all the highlights of D&D: ruins of cities, dragons, and even direct divine interference. It's also pretty long, going from 5th level to 10th level.
It has a few flaws, though they're fixable. Indeed, since the module is popular enough to have self reinforcing popularity, a handbook dedicated to helping a GM flesh out the module exists. Firstly, like most modules, it assumes a very low optimization level by players, so monsters could be toughened up, since if your party is competent at character building they'll just laugh at the rank and file soldiers. The end is also the weakest part of the module: It's a simple dungeon (instead of a sandbox) where the overall commander, who hasn't been introduced yet, awaits some pissed off PCs. Since the penultimate part is a pretty sweet climax and has the PCs defeat a general who could very easily be made into the hoard's leader, it can be chopped off with little effort. A handful of events aren't as explicit as they could be.
The 4E series Scales of War has a few elements that follow up on the plot to this module, though isn't a direct sequel.