In hard-to-mow areas, I use a string trimmer to occasionally cut back the weeds. I recently found a tool for sale which could potentially be used in a similar fashion, but without any gas. It's called a Bow-Knife Weed Cutter. I found it here, but other people sell similar tools. It looks like this:
The spring blade can be released for storage, and sits straight. I realize it may take more energy (which I don't mind) but it is also considerably lighter in weight than a commercial grade string trimmer.
How long (hours) can it be used before needing to be sharpened?
How does it compare to a gas powered string trimmer in regards to the cutting speed?
How well does it handle small woody plants (ie, roses or blackberries) that I may run across?
The customer reviews I've read seemed fairly contented, but there weren't many.