They are quite similar to KDE's calculator and color picker applets (a little bit more simple, and even partially unfinished, but usable enough to fill my needs).
Both applets are written in python + pygtk.Please note: i use neither GNOME nor KDE these days, don't ask for improvements/fixes. Just do it yourself!
calculator applet for GNOME panel: wcalc-applet-0.2.tar.gz
color picker applet for GNOME panel: wpicker-applet-0.1.tar.gz
you can use "xfce4-xfapplet" package to load GNOME applets in xfce4 panels.