What's with spelling on the internet?

Bad spelling on the internet: we’ve all seen it. pritty (pretty), rely (really), red (read), aventure (adventure), pleas (please). Since I’m assuming that most of these people aren’t using English as a second language, it actually takes effort to spell this way. Why do people do this? Is it some attempt to be cool that I’m not in on? An internet meme that won’t die? Drugs really are as bad for you as propaganda says?