lighttpd is used by many well-known sites. The typical scenario is using lighttpd as off-load server to push out static content and leave to complex work to another server.
One example is YouTube. They have a farm of servers which push out the thumbnails you see before you see the movies:
$ curl -I http://sjl-static16.sjl.youtube.com/vi/TgF_eRkfqEY/2.jpg
HTTP/1.1 200 OK
Content-Type: image/jpeg
ETag: "983726135810477085"
Accept-Ranges: bytes
Last-Modified: Fri, 03 Feb 2006 04:32:53 GMT
Content-Length: 3495
Date: Wed, 04 Apr 2007 06:49:51 GMT
Server: lighttpd-aio/1.4.11.8
As you see in the name-scheme, there are some more of those servers pushing out content.
On wikipedia they run at least 2 servers with lighttpd:
- upload.wikimedia.org
- download.wikimedia.org
While download is used to distribute the SQL dumps of the database when tend to grow above the magic 4GByte border, upload is used to push out all the images and resize them when necessary.
$ curl -I http://upload.wikimedia.org/wikipedia/commons/thumb/2/21/Mandel_zoom_00_mandelbrot_set.jpg/250px-Mandel_zoom_00_mandelbrot_set.jpg
HTTP/1.0 200 OK
Content-Type: image/jpeg
ETag: "6460328581220324712"
Accept-Ranges: bytes
Last-Modified: Mon, 04 Dec 2006 22:24:53 GMT
Content-Length: 5973
Date: Wed, 14 Mar 2007 11:16:19 GMT
Server: lighttpd/1.4.13
X-Cache: HIT from sq13.wikimedia.org
X-Cache-Lookup: HIT from sq13.wikimedia.org:3128
X-Cache: HIT from knsq12.knams.wikimedia.org
X-Cache-Lookup: HIT from knsq12.knams.wikimedia.org:3128
Age: 6265
X-Cache: HIT from knsq10.knams.wikimedia.org
X-Cache-Lookup: HIT from knsq10.knams.wikimedia.org:80
Via: 1.0 sq13.wikimedia.org:3128 (squid/2.6.STABLE12), 1.0 knsq12.knams.wikimedia.org:3128 (squid/2.6.STABLE12), 1.0 knsq10.knams.wikimedia.org:80 (squid/2.6.STABLE12)
Connection: close