Cut rope (no tools, no knife)

Pretty neat. I don't know how large you can go with it though

http://www.wimp.com/cutrope/

 

That could be handy. You can also cut plastic plumbing supply and drain pipe like that with nylon builders/masonry line.

 

I once had to slice through a snagged anchor line - used the back of the enamelled steel centreboard, which tells you how sharp it was....