You guessed it, it's filterforge, which has recently been updated to version 2. There's a free 30 day trial, and after trying it, i had to break down and purchase it. It's very cool. I'm pretty sure I can apply LGPL license to textures made with it. Some info is at the bottom of this page.
http://www.filterforge.com/wiki/index.php/10q
I Wish I had some good ideas (and talent) for base textures. I would like to make complete skin sets. If anybody has some decent artwork for that, I'd like to see it.
I don't think there is a texture on the treads. It's just a color? I'm not sure how that works because the model is hard coded into the app. The .obj format model that Jeff provided in the wiki does not include the treads as shown above. The treads.obj is something else I'd like to have, actually.
You can see the treads by changing your BZFlag settings at -->Options-->Effects-->Display Treads.
Take a look at my Defender game mode concept.
Thinking is not an automatic process. A man can choose to think or to let his mind stagnate, or he can choose actively to turn against his intelligence, to evade his knowledge, to subvert his reason. If he refuses to think, he courts disaster: he cannot with impunity reject his means of perceiving reality.