Gigapixel projects are very popular these days. They are a great challenge for digital photography and they often promote a city.
After London, now Sevilla, Spain, has been captured with 111 Gigapixels! A work of several months for José Manuel Domínguez and Pablo Pompa, who compiled 9750 photos taken with a Canon 5D MarkII. If you can read spanish, the making of is here (not translated at the time this article comes up).
As always, you can zoom in and out and turn to visit the city.
Sevilla 111 Gigapixels