The usbmount Debian package automatically mounts USB mass storage devices (typically USB pens) when they are plugged in, and unmounts them when they are removed. The mountpoints (/media/usb[0-7] by default), filesystem types to consider, and mount options are configurable. When multiple devices are plugged in, the first available mountpoint is automatically selected.
The script that does the (un)mounting is called by the udev daemon.
USBmount is intended as a lightweight solution which is independent of a desktop environment. Users who like an icon to appear when an USB device is plugged in should use the pmount and hal packages instead.