With default configuration AltGr-e (the right Alt for those who have no AltGr) produces the generic currency symbol which looks like a four legged spider. When the font of the program is changed to a ISO-8859-15 font the currency symbol is replaced by the Euro symbol. In Debian this can be achieved by adding line
.XTerm.VT100.font: -jmk-neep alt-medium-r-*-*-*-120-*-*-*-*-iso8859-15
to the file /etc/X11/app-defaults/XTerm. The fonts available in distributions and installations vary.
If AltGr-e does not work add line
keycode 26 = e E EuroSign
to the file /etc/X11/Xmodmap
Change the font setting in KControl to ISO-8859-15.
Change the font setting in Gnome Control Center to ISO-8859-15.
A better way of doing this is changing the system wide GTK+ configuration with commands
cd /etc/gtk ln -s gtkrc.iso-8859-15 gtkrc