The Assault on Black Reach koptas availability and inexpensiveness allowed for use as other vehicles, namely buggies. Below are three examples on Unite All Action blog, see post for more pictures. He includes a how-to article.
Also on Dakka Dakka are kopta buggies with big shootas (and rokkits.) Check them out here, and see the Kan Buggies he built too.On Deviant Art, grpdesign has an interesting build in this link.