Open Toonz has recently been released to a very excited 2D animation community. Based on the Toonz appliction (used by Ghibli Studios in Japan), it is only just started to be looked at carefully. There are GUI problems and workflow issues, but the free application is very powerful.
There are a variety of users who have created intro tutorials for Open Toonz, but I think the ones being created by computerartclub at YouTube are the best. Here is their OpenToonz intro video: