Motorola MOC3010 takes DTR on one side.  Optoisolator.

On the other end, it controls a triac thats rated at least
400V.  Or it drives a transistor that works a relay.
I prefer triacs where the heatsink isn't electrically hot.
In a box, you care less.

I've used parallel ports to run lights and do dimming (needs
something to trip an input on zero crossing).  Worked well
enough on a 1MHz Apple ][, I'd presume that a 2000MHz PC
could do ok as well.

Now?  I got a weeder tech (weedtech.com) DI/O module that
can control 16 relays.  (It doesn't, it just COULD).

Quoting David Laight (david@l8s.co.uk):
> > Speaking of DSL modems, I wish I had a little electronic switch that
> > I could open/close by controlling the DTR serial pin, say.  My modem often
> > loses synch (my ISP says that all DSL modems do this) and can't resynch
> > on its own, about once every week or three.  The only way to get it back
> > to life is to cycle the modem's power (!@#$ brain-damage inspired by
> > MS, no doubt).  For a non-hardwawre type, is there an inexpensive part
> > or switch that will let me do this?  The best any electronics places could
> > suggest was an X10 module which is overkill and way too much trouble, as
> > well as probably being 10 to 100 times more expensive than what I think I
> > really need...)
> 
> All you need is a TRIAC in the AC supply - which will switch mains
> from a few volts either side of ground.  You really need an opto-isolater
> though, to allow for the difference between mains neutral and 'earth'
> (or to allow for 'high-side' switching').
> 
> The required parts might even fit inside a UK mains plug!