Labels X restores the label function that was available for OS9. Not sure what else to say; if you use labels all the time (like I do) and can't stand to be without it, this is what you need if you're going to make the switch to OSX. If you never used labels before, well, you probably won't need it, but you should go ahead and download it and try it out while it's week-long demo lasts.
Anyway, the controls are quite simple: there is an option to turn Labels X on or off, and an option to show a Labels column in the Finder list view. Then there are the colors and names; 7 of each, all pre-set to the OS9 defaults. One of the nice things about Labels X is that it reads and writes the labels just like OS9 did, so if you label something on a zip disk green on your OSX machine at work and then open it on your OS9 computer at home, it'll still be green.
At $7, this little hack is quite handy for those of us who use labels every single day (I use them to color-code what files need to be updated and which ones don't at a glance; otherwise, updating my website would take several hours).