I'm creating art for a 2d pixel art game. I have 3D models which are rendered to anti-alias images. I then convert them to 8-bit. This creates some white pixels around the edges. Is there a cleaning tool, which will auto clean the images? I stumbled across this. Is it any good? Is there a free alternative? http://www.psoft.co.jp/en/product/anti-aliasing/ps.html

Render your images with a transparent background instead of a white one. That should fix your problem.

Are you rendering with an alpha channel?
Can you use that alpha channel in the conversion process?
Why are you using 8-bit images in the first place? (Especially with pre-rendering...)

There's always the option to not render the 3d models with anti-aliasing. Then there won't be any softening around the edges and since you're converting them to 8-bit anyway, it's not like it will make it look any worse.

Just turn off anti-aliasing on your render passes.