HT Games
Historical
Historical wargames are available for every era, from the U.S. Civil War to Napoleonic naval warfare. The two most commercially successful historical wargames are Field of Glory (a set of rules for ancient and medieval warfare) and Flames of War (a World War II game with three separate time periods). Field of Glory relies on a rock-paper-scissors mechanic and maneuvering key units, while Flames of War has myriad special rules for individual units, representing key philosophical and tactical differences between various armies.
Science Fiction
Warhammer 40,000 (also known as 40K) is the sci-fi version of the popular Warhammer Fantasy Battles game. Set in a far-future universe, the style is over-the-top, heavily influenced by gothic imagery and the metal scene of early 1980s England, with heavily armored Space Marines, savage Orks and blood-drenched Daemons. The rules are revised on a regular basis, but tend toward squad combat and hand-to-hand assaults.
Battletech is a venerable game about giant robots, set in the 31st century. Its background is very detailed, but the rules are somewhat dated, with a lot of record-keeping. Larger battles are unwieldy and take a very long time.
Fantasy
Warhammer Fantasy Battles is a popular and commercially successful game that has had numerous editions. Human armies often have a Germanic flavor, while other armies include Skaven rat-men, Mongol-influenced Ogres and the bloodthirsty Vampire Counts. Most units maneuver in formation, while heroes and magicians move individually, joining units when needed.
Warmachine is a steampunk-styled game, with spellcasters controlling magical robots called Warjacks. Players rely on a pool of power points to fuel their units and take extraordinary actions, making resource management a key part of the gameplay.