In my honest opinion, there has not been a decent space combat simulator since Freelancer or the end of the I-War series, and that was a long time ago in gaming terms.

Starlight Inception is the latest contender, and judging by this trailer it has some interesting mechanics already in place:

I will be keeping an eye on this one, while it is already available on Steam as an early access title here.

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Updated 2023 : note this entry was published in 2013, but is left here for archival purposes.