http://challonge.com/ supports swiss pairings and is easy to use. Doesn't have the best mobile website though if that's something you're strongly looking for.
Just for anyone else browsing this thread in the future, I've been using MTG Arena (https://mtgarena.appspot.com/) for my last couple of drafts. It handles the pairings, calculates results including DCI tiebreakers, and has a good mobile interface.
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