Multi Commander is build around extensions and plug-ins; almost all the tool's major features come from an extension.
There are a couple of different types of extension and plug-in.
Tools / Utilities Extension
These are extension that can interact with the core system in Multi Commander. They can show a GUI in it's own window or inside a tab.
They can extend the script system with more commands and add their own commands that will be shown in the menu.
For example, PictureViewer is a extension that shows its own gui; MultiRename is an extension that shows internal GUI elements within its own window; the File Checksum verifier shows its GUI inside a tab in Multi Commander, and FSFTPScript extends the script language with FTP support.
These are used to allow Multi Commander to browse archives, FTP, Registry and other resources that have a filesystem-like structure.
File Properties Plugins
These are plugin that are used to retrieve file properties of a file. For example, the Picture Properties plugin allow you to get properties of picture files so you can show columns with those properties, such as Image size, EXIF date and a lot more.
File Operations Plugins
These are plug-isn used during copy/move and delete operations. They can modify the target location of the file, change the data that is read/written or perform an action after a file operation is complete.
For example, the built-in plug-in "Auto Sort" will change the target location so that if a file called "MyFile.txt" is copied, it will place it in an 'M' subfolder under the selected target location. The AutoUnpack plug-in will notice if the user is copying a .zip file and will tell the system to copy the content of that file instead, so that the zip file is automatically unpacked.
How to get the SDK
The SDK is currently in Beta. There is no documentation at the moment but the SDK and samples are well documented.
The current Beta SDK can be found in the forum. If you have any questions send an email or use the forum.