Listingninja.com was created by Adam Thorsen to help keep track of the torrent of messages he gets every time he looks for work or tries to sell or buy something using craigslist. An email client is pretty dumb when it comes to craigslist. It just mixes your craigslist emails in with all the other emails, and has no concept of which emails belong to which listing.
ListingNinja is smart. Listingninja knows which emails belong to which craigslist listing. Never lose track of who you're replying to about what ever again!