I run nginx on my Pi for a personal site for friends, and Lighttpd on a VPS for a very small site with a few gifs and low visitors. The Lighttpd regularly goes down to like 5-10MB memory usage which is really interesting, whereas nginx seems to cache more in RAM. Either way, both seem to work extremely well for low RAM (and the slow CPU on the Pi).
I suspect that might be under represented by people using Lighttpd for serving static content, with something else like Apache serving the main page html (Ive got a bunch of clients in that configuration with WordPress running on Apache, and all the images/css/js/etc coming out of Lighttpd.)