xShunter is a console application written in C#, that can control an OpenTTD server and adds new in-game features as well as a link to IRC. It can be compared to Autopilot/Avignon, by features as well as by technology (attaching to the OpenTTD server console to control it). However, since OpenTTD introduced an admin interface with version 1.0 xShunter can also utilize that, making it possible to provide more features.
See here for an overview of xShunter components and how they are playing together in a community environment that has more than one server.
It's being tested on Windows (.NET Framework) and Linux (Mono) but should also run on Mac (Mono).
Currently it is in early alpha stage and only selected testers are running servers with it. Later it will be released open source and of course free.
Sorry, xShunter is not available for download yet.