Real Time Strategy
Glest is a 3D realtime strategy game (or RTS) set in a sort of pseudo medieval Europe. The game, as of version 2, features two factions, Tech and Magic. Each faction also has their own unit types, buildings and tech tree (ie more stuff to do later). As typical with RTS games there are resources to gather and mine.
The Battle for Wesnoth is a game that every strategy fan should play at least once. Though it is turn based, its heavy emphasis on combat keeps the game moving and avoids the boring lulls that other turn-based games are susceptible to. It also boasts a unique set of mechanics that allow for a much broader range of conflicts, far beyond the typical “kill everyone” or “survive for X minutes”– objectives that are common to most strategy games.