First of all, I just want to say that you could have found these answers with a little searching around. Discourse has an awesome search function. 
I guess that depends on your definition of “replacement”. Yes, the Sponge Project was formed directly after Bukkit’s real death (discarding the fact that it’s still around). However, it’s not a direct replacement, and is a much better improvement upon the old formula, as it were.
Most of these are already in the process of being formed. There is a link in this thread which leads to a Google Doc that lists all of the plugins to be ported.
Any other API for developing Minecraft plugins would share the goals, I suspect. I’d like to say that Sponge is the best one. 
But that’s just my opinion.