Over the years from 1926 to 1984, Route 66 had various termination points but for most of its existence it ended in Santa Monica. I've concluded that in Santa Monica there are three different end points: technical (where the road physically ended), ceremonial (where the Will Rogers Hwy dedication took place), and popular (where people talk about it ending).
The Santa Monica Pier is what I call the popular terminus. It's actually two blocks south of the Will Rogers Hwy dedication and six blocks west of Lincoln and Olympic, so it's definitely not a legitimate end point. However, it's so close and it's so much fun and it's so photogenic and it's so recognized that it's a convenient reference point. Furthermore, it's a great place to take pictures since there is nothing much to photograph at the other end points and the Pier has this wonderful neon sign. At night it glows beautifully.
I mentioned the pier is lots of fun. The amusement park is a big reason why: