I use randomizer.org or random.org - both work fine. I used randomizer more only because it's easier to use if you have more than one giveaway - you can set it up in sets and then have your number of winners.
The easiest way I have found is it use random.org and use the list randomizer. It is under fun & free. When do my entries, I have a doc. with all the names on them, then I just copy/paste everything into the randomizer, and it gives me a name or names if there are multiple winners. Hope that helps.