http://achrongame.com/Meta-time strategy, when real-time strategy just isn't enough! :V
Achron is an upcoming strategy game with free-form multiplayer time travel. Gameplay proceeds not only on multiple instances of space, but also multiple instances of time. The game is currently in alpha and is available for pre-order.
In addition to being able to play in the present, players have a limited ability to view and issue commands in the past and future. Events in the past can be changed by issuing command in the past, and the outcome of those changes will eventually be seen in the present. Their ability to change the past and future is limited by a resource called "chronoenergy" which regenerates as long as they stay in the present. Past events are also frozen after a certain amount of time has passed, and past changes take time to propagate throught he time line during which they may be countered or undone before reaching the present.
Individual units can also travel through time via "chronoporting". It's possible to "chronoport" units into the past so that they can fight alongside themselves so long as the player is prepared to deal with manage the paradoxes that result. For example the instance of that unit that will have travelled back in time must eventually be chronoported back in order to maintain changes done by the instance of that unit that already travelled back in time. Also, the player must prevent the unit from physically colliding with past or future instances of itself time travelling into the same physical space or the unit will be destroyed, or "chronofragged".
There's a series of gameplay videos demonstrating some of these gameplay mechanics:
Time Travel Explained:
http://www.youtube.com/watch?v=Kvp8FE3BiQMMultiplayer Time Travel:
http://www.youtube.com/watch?v=O6PuVyZGrYEChronoporting:
http://www.youtube.com/watch?v=z4L3J_bfjLUGrandfather Paradox:
http://www.youtube.com/watch?v=CTVsXbWQXp0Using the Future:
http://www.youtube.com/watch?v=f21y2Qkj41gPause:
http://www.youtube.com/watch?v=hgDmhIQJBkQHierarchy:
http://www.youtube.com/watch?v=OT_rXF5t72s