I started playing Achron. It's an RTS with time travel in it. Time travel has always been a fascination of mine, so games like this, Chrono Trigger, and the tabletop game Continuum appeal to me.
The idea is that there is a seven minute mutable time period where you can basically time travel with impunity. Changes made in the past are propagated to the future by time-waves. So basically you can suicide scout in the future, then cancel the order in the past, and when the time-wave hits your scout hasn't moved, but you remember what was where the scout never went. Time does keep moving forward (somehow) and as it does the past gets locked off.
I've given myself a tremendous headache trying to think in four dimensions but it's a stupid amount of fun.