Snippet Compiler has had add-in support since 2004, but I haven't had the time to publish the API or any of the samples. Hopefully these add-ins, with source included, will be a good starting point. Please let me know if the SDK is missing something useful (such as pre and post-build events).

To add an add-in, simply drop it in the AddIns folder in the Snippet Compiler folder.

Important: The assemblies and source are .NET 2.0 and therefore will only work with the .NET 2.0 version of Snippet Compiler. However, 95% of the source is 1.1 compatible and should run in the .NET 1.1 version of Snippet Compiler if compiled against the 1.1 framework.

Web References
Creates a new web service proxy class.
SDK specific: Creates a menu item and loads a new file into Snippet Compiler.

Download source | bits

Folder Browser
Floating toolbar that displays all of the files that Snippet Compiler can open. Double-clicking on a file opens it.
SDK specific: Creates a menu item,  gets the loaded languages using the LanguageManager (to filter the display), loads the selected file. 

Download source | bits

 

Settings ToolBar
Custom toolbar containing frequently-toggled options.
SDK Specific: Adds a new toolbar, adds buttons to a toolbar, creates commands, references existing commands.

Download source | bits

 

Snippet Repository
Drag and drop text snippets from Snippet Compiler onto the little window to save them. Double-click a text snippet to insert into Snippet Compiler.
SDK Specific: Addresses the active editor and inserts text into it.

Download source | bits