What is directly across the ocean from me?
Pick your own stretch of coast on the globe - or press Request Location on a phone - and every one of the 360 directions out to sea is followed to the country it reaches. On a phone you can then hold the device up and read off whatever lies across the water in the direction you are facing. The answers for New York, Miami, Lisbon, Cape Town, Sydney and 32 other well-known coastlines are listed on what's directly across the ocean.
Is "across the ocean" not just due east or west?
No, and that is the whole point. Sailing a constant compass bearing is not the shortest path across a sphere; a great circle is. Follow one properly and the answers stop being obvious: Land's End in England faces Brazil, Perth in Australia faces Africa, and a beach in New York faces Australia. Maps that answer with "due east" give a different, and less truthful, answer.
What is an antipode, and where is mine?
Your antipode is the point on the exact opposite side of the Earth - dig straight down and you would surface there. Every pick marks and names its antipode on the globe. Bearings that run past it keep going until they hit land, which is why a single direction can cross an entire ocean and arrive somewhere unexpected.
How accurate is it?
Close enough to trust for the shape of the answer, not for navigation. The analysis grid is roughly 10 km and directions are sampled every quarter degree (or every tenth, on the Detail setting), so very small islands and very narrow straits can be missed or merged - see the note above for the full caveats.
Where does the map data come from?
Country outlines and the Terrain map are Natural Earth 1:10m (public domain); the Satellite map is NASA Blue Marble (public domain). Everything is computed in your browser from those outlines - there is no server doing the work.
Is it free, and does it work on a phone?
Free, open source under the MIT licence, and no tracking of any kind. It installs as a web app on phones and desktops and keeps working offline once loaded.