A number of Sins of a Solar Empire fans have gone out of their way to create what might be the coolest crossover mod ever, titled Mass Effect: Dawn of the Reapers. Yeah, that name says it all.
Dawn of the Reapers essentially turns this epic sci-fi strategy game into a Mass Effect experience, by making its various races and ships (not to mention tech) playable in Sins' 360 open-space environment. Even the UI has been altered to better emulate the Mass Effect feel.
The mod seems to have been in development for some time and still hasn't been released to the public. It was delayed when Mass Effect 3 came out, so the creators could make some changes to specs and lore.
Whenever Dawn of the Reapers does land, however, it'll be compatible not just with Sins, but the Rebellion and Diplomacy expansions as well.
UI Test - Mod DB