I love TextPander.

My abbreviations include:

cde@ = my full email address (cde are my initials)
[two hyphens] = [em-dash]
[three periods] = [ellipses]
teh = the
libid = [my library card id number]
ustid = [my Univ. of St. Thomas id number]
wwwst = http://www.stthomas.edu/ [the domain for websites at my work]

I also use it for custom signatures on my email messages:
ustsig = [signature for internal messages (ust=Univ. of St. Thomas)]
profsig = [signature for external messages (prof=professional)]
farsig = [signature for messages re: Fine Arts Review committee]
etc.

I also use it for the entire text of "form letter" emails I have to send out occasionally.

I also have a scanned version of my signature, which I can paste into word-processed letters with [fullname]sig.