Location - On the Thonburi side of the river, opposite Wat Pho. It can be accessed by catching a River Express to Tha Tien and then taking a cross-river ferry. This drops you right outside the main entrance. Google Map location.
Wat Arun means The Temple of Dawn and is named after Aruna, the Indian God of Dawn. It is one of Bangkok's most famous landmarks and provides a great view from the other side of the river. You can combine this with a visit to Wat Pho, it is just across the river and there are small ferries that go back and forth across the river. Once in Wat Arun you can climb up around halfway and this gives you great views back across to Wat Phra Kaew.
Wat Arun is decorated with bits of broken porcelain which was orignally used as ballast in Chinese barges entering the city.
Click here for the Wat Arun website.