USB/Ethernet to a (real) bell based on a signal
Personally I'd go about this using a small PIC microcontroller or similar.  The USB side of it can be quite a pain to program (on the PIC chip) for the beginner (I haven't fathomed it yet), so a USB to RS232 converter (either as a USB dongle or as a USB to RS232 chip) can make things an awful lot easier to handle.

The PIC16F88 is a good starting point if you don't want to program the USB yourself,  and is dirt cheap (you can get free samples from the manufacturer).  If you want USB then you should look at the PIC18F series of microcontrollers (I just got my first one working today - yippee).

If you don't want to go about building your own hardware and programming the PIC yourself there are many DIY kits that come with a pre-programmed PIC (such as the K8055 from Velleman) which often have the USB built into it already.