The always amazing Perfect Captain has released A Coat of Steel, a set of free wargames rules for the Wars of the Roses. There’s also a campaign game called “A Crown of Paper.”
Beyond the Saale is a set of campaign rules for the Seven Years War.
Running a “campaign” can add context and interest to a miniature wargame. Here’s a Tobruk Desert Campaign for Flames of War.
Valley of Fire is a free campaign system for the Northwest Frontier. The authors write:
This is a fictional colonial campaign set in the Northwest Frontier during the late 1880s. A British fort is under siege and a relieving force has exactly one month to break the siege! The villages in the valley and mountain passes go into revolt, adding more and more foces to the siege and to block the British advance. The system features hidden movement, is of moderate complexity, and is designed to be run with or without a referee. The PDF contains rules, counters, a planning map, and log sheets.
The Napnuts site has a set of rules for conducting a Napoleonic Peninsular campaign. The Peninsular Campaign makes for a good campaign because of its relatively limited scope.