It has been a long-standing tradition to creatively misspell words during electronic communications. I've seen it used in much the same way since the early 1980s. Since the users of this form of communication often consider themselves to be "elite hackers," or 1337 H@(K0RZ, the form of writing is often known as 133t, L337, or something similar.
The following applet randomly shifts between various 1337 spellings at up to 30 times per second. Simply type the text you want translated in the upper text box.