http://medibangpaint.com/en/
It looks interesting with its cloud features allowing multiple users to work on the same thing at once (like one person sketches, another accesses the file and linearts, and so on) and its arsenal of comic tones and stock background scenes.
I'm already using Clip Studio Paint which also provides toning tools/stock background/tone effects, but Medibang Paint looks more extensive.
Second, Medibang seems like a great place to publish art/comics since it's very similar to Pixiv but offers international language support. You can upload your stories in .PDF format or something similar and also enable online purchases of the comic/publishing on 3rd party sites. Too bad it doesn't seem like a very popular social art site... The only time it gets any notice is its annual shounen JUMP contest.