Hard to tell.
Including Protheans and Forerunners
and Mass Relays and Rings (LOL)
Edit: Precursors too...
Edit 2: ALL OF THE STUFFS IN THE UNIVERSES EVEN TREES
[Edited on 03.25.2012 2:43 PM PDT]
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Mass Effect has the Crucible, Halo has the Halo rings. Inevitably, both of these would end up being fired and everything, organic or synthetic, would die. Unless some of the creatures on the Halo side were able to get into Onyx, in which case I'm not quite sure how that would work out.