Frozen Synapse is a high-profile indie game of Simultaneous Turn-Based Tactics. It was developed by Mode7 and released in May 2011. Frozen Synapse takes the core gameplay of other Simultaneous turn-based games such as Laser Squad Nemesis and focuses on creating short but extremely intense burst of tactical combat. The game comes with a fairly large campaign and fairly good AI, but also has a very robust array of Multiplayer features, including extremely well-done PBEM functionality.
Frozen Synapse comes highly reommended and is available straight from the developers or via Steam and other outlets! http://www.frozensynapse.com/
NOTE: These videos were recorded for the purpose of showing off the action and high intensity of a game of Frozen Synapse. They do not show the planning stage, as they can be tedious to view without added comment though voice-over.