Recipes

Not through the Plugin API unfortunately, but you can manually edit the game database file. The recipes are stored in the definitions.db in the "commons.jar" (it's an SQLite file). You'll need an SQL editor to open and edit this file.

If you want to modify the recipes in multiplayer, you have to edit this file on the server and the client - otherwise the client runs into issues.

Yes, you can edit it, but make sure to extract it from the commons.jar first (basically the .jar file is just a zip archive), then - after editing the database - pack it to the commons.jar again.

Quote from Ash_lee

its asking me for an encryption passphrase...

Which one, the commons.jar? We didn't put any encryption on that file. Just open the file with a zip program (e.g. 7-Zip or WinRAR), navigate to "commons/definitions/" and extract the "definitions.db".