Hercules mono graphic adapter
This nice vintage piece of hardware can be very usable. Unfortunatelly, by ISA bus starvation (there are no new motherboards with ISA slots for several years), the usability is still harder to find. Best thing about this solution is price - you can get Hercules card and monitor for $10 in your local computer disposal and of course kinda hackiness of such dual-head solution. Another good reason for installing Hercules is second parallel port included on most of the cards.
Probably best use is to have second textmode console with some constant
Messing with X server
Unfortunatelly, you cannot use thing mentioned above for X servers - once you start one X server on your VGA and second on Hercules, first one goes blank. (I've tested this years ago, so if this changed, please get me informed). Only option is to use Xinerama. The bad news are that Xinerama is unable to mix different pixel depths in one server, so you've got to degrade your VGA to mono adapter. For more info using Xinerama, Google it yourself :-)