The Troll
A sad individual who lashes out at anyone, for the sole reason of provoking a response.
Troll comes from the adjective Trolling.

The Rager
In some ways, a variant of the Troll.  Can be found promoting the virtues of Open Source Software, when compared to the likes of Microsoft.  Can often be spotted by the use of words like Micro$oft & WinDoze.

Misguided, aka “I pity the fool”.
Responding to a posting, where they don’t have all the facts.  Often these are newspaper readers.

Unhappy customer
Not that you’d see these on a normal blog, but you can spot these characters as they are having a bitch about a product or service.
Such as Europcar.

This idea for this post came from the US Air Force Web Posting Response Assessment flowchart.