Unlike you, I am not really going for particularly "low carb", but rather "high protein". I also have mild egg and milk allergies, so I decided to go with a pure whey protein without lactose or egg protein added in, as many powders have.
I use ON (Optimum Nutrition) Platinum Hydro Whey, vanilla flavor. It has 30 grams of protein in each scoop and 2 grams total carbohydrates (0 grams sugar). the ingredient list has a few "unpronouncables", but overall is much less scary than some of the other options at GNC.
I really, really like it and consider myself lucky to have found a good protein powder with relatively few "misses" in the process. That being said, people have really strong opinions about their protein powder and this may not be a winner for everyone.
Other people on the forums have suggested getting plain (unflavored, unprocessed) whey from a dairy or natural foods store. I have heard that this is much more cost effective and just as beneficial, but I am unsure of the specific nutrition information. It may be worth looking into, though...