How to resuscitate a completely flat battery?

My car battery (2 years old) has completely discharged, partly because I don't use this particular car very often, but more likely because I may have left an interior light on.

I have given it a full fifteen minute charge from another car but on turning the switch (the other engine still running) there was not a flicker of power. There seems to be no way of checking whether there is enough water in it.

Is there hope yet for this battery, or should I replace it?

kisoro
January 2012