This is the first in a series of articles about the state of FOSS software in terms of UAVs. We start with the place where I have the most experience, the Parrot AR.Drone.
On the client side, Parrot has released a complete SDK for controlling the AR.Drone. It includes support for the control protocol, the navigation protocol, and the video streams (which is handled by ffmpeg).
However, many people choose to make a clean implementation based on the SDK docs. This includes my own UAV::Pilot, and also NodeCopter, an implementation for Node.js. Both of these are under a BSD-style license. The parts of UAV::Pilot that interface to ffmpeg are LGPL.
Control is done over WiFi. It will create its own ad-hoc network by default. With some trickery, you can connect it to an open access point, though it doesn't seem to support WPA authentication.
The on-board electronics for the AR.Drone has a Linux system with Busybox, which you can telnet into. This seems to be mostly useful for debugging the commands sent over the network, or configuring it to connect to an open access point. Hacking it to control external devices via USB seems like it should be possible, though I haven't been able to find anybody who actually accomplished this. Just a lot of people talking about it. C'est la vie.
Version 2.0 of the AR.Drone includes front and bottom cameras, which can stream 720p video. They're on par with cheap webcam quality.
As I've complained about before the AR.Drone's SDK doc is rather poorly documented. The control protocol is mostly fine, but there are a lot of missing details about the nav data and video protocols. The information you need is spread out on the dev fourms, and it just shouldn't be that way. I'm also dealing with the multiconfig options, which requires getting the ACK bit off the nav data stream, which seems all backwards to me even *if* it was well-documented. The config should have been handled over a TCP stream, especially when multiconfig was implemented.
It is relatively cheap, though, at $300. A basic ArduPilot setup will be around $700 if you include the telemetry module and a cheap 6-channel radio. That setup wouldn't even have a camera.
All things said, I think the AR.Drone is a good way to get started considering what's out there right now. But it could definitely be improved.