I only recently found out about the Unity Dialog Engine. What other third party tools and components are there available for Unity? Are they free, commercial or open source? How well supported are they?
They are available on a show-logo license and alternatively a paid license. Example projects with all source and resources are also available for both projects.
For steering AI, UnitySteer would be your match - a port and adaption of OpenSteer, this open-source project will run on any configuration - in any Unity version. This project also comes with examples. OpenSteer is open source on an MIT license.
For networking you can take a look at the SmartFoxServer API for both regular Unity as well as iPhone. The Unity API comes with 4 tutorial projects, sources for the tutorials as well as the API itself.
You will need both a server (found here: www.smartfoxserver.com) as well as the client API for Unity (here: http://www.smartfoxserver.com/labs/API/)
Also you might want to check out these two Unity Summer Of Code 09 projects:
Here's a nice list:
There are many small and large scripts donated to the community on the Unify wiki
For generating Terrain and Splatmaps from sliders or height map,
with seamless unity integration: http://www.starscenesoftware.com/Fractscape.html
Terrain is then customizable in unity.
Agon and OpenFeint social network platforms for iPhone platform.
photon and neutron are two networking solutions for unity games. photon is a fast game server and neutron is another type of game server with less customizability but more built in features. cmune platform is a platform for building social games with unity.
I just also discovered a good list at danim.tv http://danim.tv/blog/archives/unity3d-tools/