I was just reading through a great series of blog posts by Oskar Landman on System Center Central on writing a management pack for a NetApp environment.  This is an example of a custom management pack using SNMP.  Even if you don’t need to monitor NetApp, this a great detailed walkthrough of creating a management pack to discover and monitor SNMP devices. 

This is pretty advanced work, so you need to be familiar with building modules and custom workflows.  These concepts are covered in the Composition section of the MP Authoring Guide if you aren’t familiar with them.  You’re also going to need to know concepts of building and discovering classes and relationships which are covered in the Service Model section.  While the classes and modules used in this example uses are not specifically discussed in the Authoring Guide, all of the concepts of designing classes, building discoveries, and creating custom workflows still apply.  Oskar does a great job of walking through the details of the MP and illustrating these concepts for a non-Windows environment.