Hugely useful. You can search and book so many things this way. Plus cool links to tools I use a lot like great circle mapper and wheretocredit. This is seriously all you need to be a frequent flyer.
This plugin enhances ITA Matrix by adding new features such as booking links to airlines, online travel agents, and meta search engines
It also integrates with Where to Credit, Seat Guru, Plane Finder, and Great Circle Mapper