Well, so far, online:
ISTJ - Perpetual inane thread generation.
INTP - Vague open-ended comments that are usually humorous. They may be inconsistent posters that appear and disappear unless the subject is notably interesting to them. Very passive.
INTJ - Expounds on an area of interest if someone accidentally stumbles over one...like land mines. They might also argue just to ruffle the feathers of types that are easy to ruffle.
ISTP - Will usually claim to be (or possibly even believe they are) an INTP because they get no respect otherwise, but can't hack the intellectualism and resort to being tin despots and annoying the hell out of the real rationals. How can you tell the difference? ISTPs are on way more than INTPs and the content of their average post will be vacuous chit-chat.
INFJ - Will try to understand all sides and smooth things out.
INFP - Makes an effort to be uplifting, and generally has strong views of how things should be to make everyone happy all the time.
ENTP - Will blather endlessly about their lives and loves, because nobody actually cares in person. They are much funnier in person than they are online.
ENFP - Bursts in, everyone loves the comments and goes wild, then they disappear. They do the same thing in meatspace.
ENFJ - Thinks they can smooth things over, but loves to argue with rationals in spite of complete inadequacy for the task. Almost invariably resorts to create straw men in an attempt to fight these much-hated rational opponents.
ESXX - Almost never post or IM, but you will find them on all the hip social networking sites like MySpace, Livejournal, Xanga, etc.
ISFX - Obvious because they are completely and hopelessly unable to follow a conversation or logical comment the first time. Expect them to respond with a totally off-the-wall comment about hurt feelings based on a misunderstanding when you were discussing something completely different. They do the exact same thing in person, making communication exceptionally difficult and unrewarding. I avoid them, including my mother.
ENTJ - Insufficient data.