Actions performed by the writer are often wrapped in smaller-than- "<" and greater-than- ">" signs:

<g> Grinning
<gg> Big grin (etc.)
<BG> Big grin
<eg> Evil grin
<ig> Impish grin
<dg> Devilish grin
<s> Smiling
<ss> Big smile (etc.)
[s] Smiling sarcastically
<g,d&r> Grinning, ducking & running
<grin>
<chuckle>
<smirk>
<smile>
<sheepish grin>
<silly grin>
<wink>
<lewd wink>
<frown>
<raising eyebrows>
<arching eyebrows>
<<mounting the soapbox>>
<<falling off the soapbox>>
<<ducking>>
<weg>-wicked evil grin

*blush*
*hug*
*kiss*
*wink*

Some Others:

<!> An important part of a message
<?> One part of the message contains a questionable item
<ahem> clearing your throat just before saying something
controversial
<sp?> word/name/phrase probably contains a spelling error

 
CompuSlang--is a dialect that constantly grows and changes, although some of the words and phrases have become standards.

Drop carrier--To disconnect without logging off properly.  Dumped Involuntary victim of a carrier loss.
Flame--To post a disparaging or controversial message.
Lurker--Someone who reads, and "listens in" on discussions without participating or contributing to libraries.
Netiquette--Accepted practices on a service net.
Thread--A series of messages or conversations that follow a single thought or topic.