datapile uses Sinatra
Scalatra (we had to pick Sinatra on StackShare since Scalatra is has not yet been officially added) is the slickest (not to be confused with Slick for Scala) web/applet server framework we've had the pleasure of playing with in Scala.
For all intensive purposes, Scalatra is the Scala version of Sinatra (which for anyone who doesn't know is a Ruby web server DSL).
Karma uses Sinatra
We use Sinatra a lot. I love Sinatra for APIs. It's really simple, really lightweight. It's awesome.