For example I wrote a self-hosted comment-system, which is like a very basic disqus service. The back-end is a simple sinatra service that just allows "GET /comments" and "POST comments". (http://github.com/skx/e-comments/)
Last week I wrote a webhook-consumer, again using Sinatra. It receives a POST from github, parses the received JSON, and adds some details of the body into a queue for later processing.
Both services allow simple deployment and total may 50 lines of code each.