As a fan of space combat sims, my gut twisted as I watched the massive battle between the Reapers and everyone else play out via cut scene in Mass Effect 3. I wanted to control it. Phoenix Interactive's Mass Effect Reborn mod for Homeworld 2 gives me that chance.
Mass Effect Reborn, released in early beta earlier this month with a planned full release in late April or early May, gives Homeworld 2 players control over the forces of Earth and friends, as the ancient Reapers arrive to kill everything kill-able in the universe.
I love it. I wish game developers would do this sort of thing -- realise that their adventure game couldn't handle say, a fistfight and tag in a fighting game developer to make it happen. Mass Effect's engine not built for a universe-rattling starship skirmish? Don't worry, BioWare, Phoenix Interactive's got this.