Tilly’s Very Bad Day is a set of free wargames rules for the Thirty Years War. They are designed as a fast play set.
This site has a good history of the Thirty Years War, with lots of of information about the various phases of the war — The Bohemian Rebellion, the Palatinate, etc.. Lots here that I never knew about.
If you’re interested in gaming the ThirtyYears War, there’s a post here about a set of rules for the period.
Tercios Brevis Editio is a free version of the Tercios rules for the Thirty Years War.
Gott Mit Uns is a set of free wargames rules for the Thirty Years War. The author writes:
Gott Mit Uns (GMU) is a tabletop miniature wargames simulation of the Thirty Years War (1618-1648) that lets players refight the largest battles and seiges in a historic and enjoyable way, from White Mountain to Norlingen and beyond. GMU can be played with two players, one per side, but is more enjoyable and managable as team game, with one player per wing per side (1-4). The best miniature scale for the game is 10mm for visual impact, or 15mm. A 28mm game of GMU would require a very large space.