The Troll -- Everyone knows this guy (and it's usually a guy), who intentionally visits sites in order to stir things up, provoke a furious reaction from other posters and then disappear. Classic examples are the Free Republic types who visit Daily Kos and vice versa.
The Skimmer -- The commenter, usually sour, who reads only a headline or sentence of a piece, draws exactly the wrong conclusion, and then embarks on an embarrassing rant.
The Kumbaya -- These folks always show up two-thirds of the way through any heated on-line debate and ask, "Why can't we all just get along?" They are inevitably ignored or trashed.
The Parser -- What would we do without the Internet Grammar Police? These folks assiduously correct the online writings of others -- never quite catching on that the Web is designed to be fast, fresh and sloppy.
There are a couple of dozen types identified altogether. Link -via Geek Like Me
(image credit: ABC News)