This might be an oldie to some of you, but I hadn't seen it before so I thought I'd share :-)
One issue I have is that data in MySQL is taking up a HUGE amount of space, and I'm about to have to upgrade my entire dedicated server just to get more storage! But then I came across this:
$string = "example";
// compress the string
$compressed = gzencode($string);
// decompress the compressed string
$original = gzdecode($compressed);
I took a string that was 1390 characters, and compressed it to 723 characters!
It actually does a poor job with short strings like in the example, but if you have a message board or private messages where the text can get quite long, that's a different story! From my basic testing, you'll save storage on anything over 58 characters.
Only downside that I can find, when it's compressed you won't be able to use MySQL to do a simple search in the field :-( I tried using gzencode() to compresses the search term and then used strpos() to see if the compressed string contained the compressed search term, but it wasn't found. So it's not a great solution for me to use with a message board (unless someone can suggest something better), but it would definitely help with Private Messages.