My first thought was definitely SoftwareApplication but everything about Google's documentation on the Schema.org type seems uncertain (except for when it comes to apps or games). At the top of the page they say:
We are currently piloting this feature with a restricted set of initial data providers. We hope to soon open up the feature so any provider who implements spec-compliant markup is eligible to participate.
Their examples of how to implement the markup also include an application category that doesn't exist.
So after seeing all of that, Product then seems like the best Schema.org type but it doesn't feel like the best representation of the product; I've never seen Schema markup for a plugin. Does anyone have experience with Schema and plugins?