I don't think this source has been posted so far. Under: https://www.archion.de/en/
you can find digital copies of protestant church books from Germany. It's to my knowledge by far the largest site, it's similar to Ancestry just that this is run by the church itself I think. It's a paid service!
Here's a success story as published in their newsletter (in English):
In this success story a guy could finally overcome a brick wall from his 2G Grandparents who were from Germany.
EDIT 10th of April 2016: I strongly suggest to first go and browse the collection before you sign-up. That being said, they are very fast adding church books from all the federal district churches (Landeskirchenarchive). The ones in green are already added and some others might be still under German privacy rules and thus even though they are already shown in the index it's not possible to access them.
The quality of the scanned documents is pretty good, very high quality as this project was just started 2 years ago I think and they are using the latest technology.
So in summary, if you know roughly where in Germany your ancestor were coming from before they emigrated you have a very good chance to find their origin and details in the church books. As was said in the newspaper article, it's usually just a couple of pages per parish you have to go through to check for the right name (within the relevant time period).
This is obviously a great chance to overcome the usual transatlantic brickwall and go very far back in time (some German church books started in 15xx).
Disclaimer: I'm not related in any way to Archion of whoever their mother company is. I also haven't used their paid service but I checked some pages during their beta test when access was still free. Unfortunately any church books of interest for my ancestors weren't online back then.