Cigarette butts are the most commonly reported item in litter counts. That is partially because they are so small and easy to think that tossing one away will make no difference, but also because they last so long (up to 15 years, and even then they don't degrade).
Greenbutts is a new filter that will biodegrade in a few weeks. This could be a very good thing for addressing litter.
The people behind the filter plan to have it on the market by 2014.