IMHO= in my humble/honest opinion (i had problems with this one at first)
OP= original poster--the person that started the thread
PM= private message, the SDN equiv of an email. click on the icon with the people shaking hands to send a member a message that only they can read.
i'm sure there are more, but these are the three that came to mind right away. many of them are just common internet vernacular that have carried over onto SDN. feel free to pipe up with others...