Slackware 13.37 uses XFCE 4.6 as one of their supported DE besides KDE. There are a lot of small DE supported in Slackware, but KDE and XFCE are the most widely used DE currently (besides GNOME from GNOME SlackBuild Project).
One year since the last stable release, XFCE has been working so hard to improve and they have released two major releases since then, 4.8 and the latest one, XFCE 4.10 which was released yesterday (the same day as my birthday, April 28).
Robby Workman has been working on XFCE for some time and he offered XFCE 4.8 in his repository for those who wanted to test XFCE 4.8 on their machine, but it wasn't included in Slackware-Current until today (KDE made it to Slackware-Current two weeks ago).
Right now, XFCE is at 4.10 and i have a strong believe (and so does other people in LQ) that next Slackware will also include XFCE 4.10 and replaced the old 4.6.
To see what is the new features in XFCE 4.10, have a look on XFCE 4.10 Tour, and if you wanted to see the detailed changes, i would suggest you to look on the ChangeLogs.