Military.com. It's free, but you have to register.
It's where I look up various names in news articles and also how I detemined who the 4 troops we adopted were as far as branch of service (BoS). Just basic info like branch of service, pay grade, etc.
I'm not sure it's foolproof. I tried to look up some people I know who've been in the military more than 10 years, and they didn't show up in my search. For what it's worth.