Welcome to the Sponge project!
The first question on your mind: Where do I download this? The answer: you can’t. 
OMG! I know! This is a new project just over a week old.
That’s ok. You copy your plugins over to Sponge when it comes out and it’s all smiles. No. Plugins need to be ported, ask your Dev to port the plugins. Sponge also supports Forge mods. I will edit this post frequently as needed. Keep watching this thread.
Hope this helps.
ButterDev
PS don’t tl;dr