I bought some ground beef around 2 weeks ago and used half of it, putting the rest in a freezer bag in my fridge (not the freezer).  It has since turned brown (not red, as when I bought it).  Is it safe to use to make hamburgers? 
I'm not going to comment on whether or not it is safe, because that could be any number of issues other than the brown coloring of the meat. However, the brown in and of itself is not an issue. When meat is exposed to air it turns a brown color. This does not effect the falvor of the meat, but the color turns a lot of people off. Grocery stores will actually throw this meat away, not because it is bad but just because people won't buy it. Terribly wasteful.      From the USDA: Optimum surface color of fresh meat (i.e., cherry-red for beef; dark cherry-red for lamb; grayish-pink for pork; and pale pink for veal) is highly unstable and short-lived. When meat is fresh and protected from contact with air (such as in vacuum packages), it has the purple-red color that comes from myoglobin, one of the two key pigments responsible for the color of meat. When exposed to air, myoglobin forms the pigment, oxymyoglobin, which gives meat a pleasingly cherry-red color. The use of a plastic wrap that allows oxygen to pass through it helps ensure that the cut meats will retain this bright red color. However, exposure to store lighting as well as the continued contact of myoglobin and oxymyoglobin with oxygen leads to the formation of metmyoglobin, a pigment that turns meat brownish-red. This color change alone does not mean the product is spoiled  