I’d like to add to this CarrotShop, which in my opinion is the best shop plugin on Sponge at the moment. It’s a lot like Bukkit’s SignShop plugin, creates very custom shops which can support all kinds of meta and NBT data. It has a large range of types of shops including admin shops, redstone function shops (great for casinos and stuff), etc. It also comes with a report system, which creates a webpage with everything people have bought or sold at your shop, which in my opinion, is seriously cool and really useful.
I’d also like to recommend one of my own plugins, DTPunishment, it is a great tool for server owners and thier staff. It adds a unique point activated automated punishment system, so scaling punishments can be done fairly and consistently; Staff just add points for offences and the plugin gives the correct punishment based on the points that player has. It also comes with a chat filter for caps, character spam, swear words, etc. All highly configurable.
And like Simon, I recommend using the Forge version of WorldEdit over the Sponge version, and AdamantineShield over Prism.
I also personally use PixelAutoMessages for server automessages.