Imgur. Over a million pageviews a day, built on PHP, Kohana 3, memcached and MySQL. I used to use my own custom thing instead of Kohana, but it ended up being more trouble than it was worth, so I just recently switched it over.

目前还没有被墙的图片分享网站，之前有见人用过，界面还算不错，可惜中国地区无法使用，他们遮蔽了中国地区的 ip。

http://imgur.com/