Dropsix
ArboristSite Lurker
I used the billy goat walk behind a few times. Works pretty well. Doesn't leave a very clean path as it doesn't cut very low (depending on what type of area you're working with), but it does the job.
http://www.billygoat.com/Product-Categories/Detail/bc24-series-outback-brushcutter
http://www.billygoat.com/Product-Categories/Detail/bc24-series-outback-brushcutter