Out of curiosity, I went to a site which I knew ran Ruby on Rails and gets a lot of traffic, and inspected the HTTP headers.
Unsurprisingly, the response when requesting the main page included this header:
Server: Apache/1.3.33 (Unix) mod_deflate/1.0.21 mod_fastcgi/2.4.2
However, as the browser started requesting images, they came back with this header:
Server: WEBrick/1.3.1 (Ruby/1.8.2/2004-12-25)
It seems like they’re using the so-called not-for-production webserver to serve up static content. That’s interesting, given that some have described the usage of that server to be reserved for cases when you have low concurrent users and mostly dynamic content. This qualifies as neither.