Side view.

The people who really know what they're talking about will say it's hard to identify by sight alone. But it looks alot like my Liberty apples. It's a macintosh relative with a little more tartness than macs. It could be another apple also.

Definitely looks like it could be Liberty- That good, rich, deep red (almost black) recalls the heritage of Liberty apple. I think it is heavy in Macoun, Detroit Black, Fameuse- so dark on the outside, bright white on the inside (although often streaked with red inside in my experience.)

If it's really tasty and a mediocre keeper, ripens midseason and is remarkably disease free, but attracts codling moth, it could well be Liberty.