The Joshua Project isn't exactly a missions organization, in that they don't (as far as I can tell) actually send missionaries. They do work with missionaries, though. The Joshua Project maintains a list (though list is too simple; maybe more a catalog) of ethno-linguistic people groups. By identifying which groups have never heard the gospel, they help missionaries and missions organizations know where there is a need. They help missions groups develop their vision and goals.
Also, they encourage prayer for unreached people groups. If you subscribe to their updates or use their widget, you will have a different unreached people group to pray for every day. The unreached people group of the day is here: http://www.joshuaproject.net/upgotdfeed.php