I always got a chuckle out of 1337 5934}. If you don't know what that is, try piping it through the command below.
In my opinion, a lot of people have cluttered 1337 5934} with bad ideas or ambiguous notation over the years. IMers are the worst. In fact, while I get a chuckle from 1337 5934}, I severely roll my eyes at IM-speak. It's probably because people who use IM-speak are just being sloppy because they're too lazy to press a couple extra keys (something I have little sympathy for since I spend more time using a computer than most people spend sleeping), while people who use 1337 5934} do so with a combination of humor, meticulousness, and pride (however ill-founded the pride might be). 1337 5934} should be an unambiguous translation of one letter to one symbol using symbols that look as close as possible to the original message while leaving as few letters as possible untranslated. I don't really know what IM-speak is besides lazy and unreadable.
That said, this is my favorite version of 1337 5934}, written as my favorite command for doing the translation:
tr 'a4b8c<d[e3ffg6h#i!jjk}l1mmn~o0p9q@r/s5t7uuv^wwx%yyz2' '4a8b<c[d3eff6g#h!ijj}k1lmm~n0o9p@q/r5s7tuu^vww%xyy2z'
This is subject to change, but probably only if I think of a reasonable translation for the letters fjmuwy.
(Hint: many of these symbols look closer to the capital letter.)