Freebie time again. I found the blog, Brass Goggles, again the other night. I have quite a soft spot for all things steam punk, it suits my anachronistic nature. So in that spirit, I went to my library of brushes and found my cogs. I had developed a set of gears for a flash project that is still in development, and several hours of research is still necessary but this is an interim stage, that I would like to share.
In order to use these brushes, just open the file in illustrator and apply brushes to various circles and drawn lines. As illustrators brush engine doesn’t scale all that well, I recommend using trial and error to your settings. Feel free to pull out the pieces and edit them, just please don’t redistribute them without attribution.
Thanks, Mike.
Update: Hey folks, it seems there has been a few issues with how to set up the cogs. This is as much my fault as anyones, as I use my own technique instead of the official Adobe way of creating Brushes. Illustrator can be a bit snarky about bends and turnarounds, and as strange as it sounds, loading brushes backwards solves the problem in some cases. So here are some usage hints:

Start a new doc, creating a new ellipse to begin. I use arbitrary settings so that they can bite me in the ass after several screen caps.
Apply your brush to the ellipse. I choose the first one as I have experimented on it extensively as a proof-of-concept.
It is worth noting at this point that most of your questions start here. So this is where I didn’t provide enough info.

The fix is this: Use the brush options button to open the options dialog. This is where you can exert some power on the applied brush(make sure one is selected!).

Consequently, this dialog will appear:

Notice that it comes pre-scaled at 20% and I have the cursor over “Flip Across.” If the cog looks inside out, this is the setting you need to invert, ie. if it is flipped, unflip it. Notice my teeth are on the outside now.
Next, we need to make this cog solid, in order to use another brush inside it. So I will go ahead and create a .8″ ellipse inside it also.(It doesn’t need to be centered yet, I have intentionally offset it here to illustrate which piece is which.

Note the second is not much smaller, which is also to show how other brushes interact. Now we apply a brush to it, using Pattern Brush 22, which I like. It has a large center section and a bit of a curve to it.

Wow, that is really screwed up, but wait! You know how to fix it. That fun Brush Options dialog! I set mine to 30% initially with Flip Along Unchecked, and Flip Across Checked. Yielding this:

If you are unable to color your brushes, it maybe because of Hue Shift is not the option selected at the bottom.
Now it is up to you to make it nice. I did finally settle on 40% for my scale, and used the align palette to center the two original brushes and the new little white circle that I created. This is the final example:

If you are still having problems recreating the file, the above image is a link to download the source file.
Update no 2:Illustrator 8 Version
Good Luck!