Lluis just announced Mono.Addins:

Mono.Addins is a framework for creating extensible applications, and for creating libraries which extend those applications.

Mono.Addins has been designed to be easy to use and useful for a wide range of applications: from simple applications with small extensibility needs, to complex applications which need support for large add-in structures.

This new framework intends to set an standard for building extensible applications and add-ins in Mono.

The main features of Mono.Addins are:

• Supports descriptions of add-ins using custom attributes (for simple and common extensions) or using an xml manifest (for more complex extensibility needs).
• Support for add-in hierarchies, where add-ins may depend on other add-ins.
• Lazy loading of add-ins.
• Provides an API for accessing to add-in descriptions, which will allow building development and documentation tools for handling add-ins.
• Dynamic activation / deactivation of add-ins at run time.
• Allows sharing add-in registries between applications, and defining arbitrary add-in locations.
• Allows implementing extensible libraries.
• In addition to the basic add-in engine, it provides a Setup library to be used by applications which want to offer basic add-in management features to users, such as enabling/disabling add-ins, or installing add-ins from on-line repositories.

If you are interested in knowing more about Mono.Addins, this document explains the library basics and some examples see our Introduction to Mono Addins document.

And in-depth explanation is in the Mono.Addins Reference Manual

This is currently available from: here.