Is my ground beef still good?

The meat is fine. The part you cut was most likely brown when you put it in the freezer. We grind meat daily at my shop and by the end of the day anything we ground in the morning is turned that brown color on the inside, and bright red outside. It's just the way it happens.. don't' be concerned. Now if the meat is closer to a greener shade, that's when you throw it out. Also, if you're still not sure, defrost it and smell it. If it smells a bit sour, toss it. But it should still smell like regular ground meat :)