XfApplet is a small program that allows you to run Gnome panel applets from the XFCE panel. It's available in the Ubuntu universe repositories. You can install it from the command line using aptitude. (You can also use apt-get.)
If you're not using Ubuntu you can download the XfApplet source package and compile it for your system.
Once the installation is complete, right click in the XFCE panel and select Add New Item. Scroll through the list and select XfApplet and click Add. A pop-up will then come up where you can select which Gnome applet to add. I chose Time Tracker so I could use the Hamster Applet from XFCE. You can only run one Gnome applet in an instance of XfApplet, but you can run multiple instances of XfApplet to run all the Gnome applets you want.