The lightest ascenders are the ones that (almost) don't weigh anything: knots. Originally "everyone" used knots, but they were eventually supplanted by mechanical ascenders that work better in the most common situations. I still think people should learn knots for several reasons. First, they are less forgiving, and when someone learns the proper balance to make knots work well, this will improve their form and efficiency with mechanical ascenders. Second, knots can be improvised when the mechanical gear is not available. Third, knots can "always" be made to hold. Finally, they are fun in their own right.
Bob Thrun is the published expert on ascender knots, and his book Prusiking is excellent reading if you can find a copy.
The Gallery shows the following knots:
