What could cause the air conditioner on my 98 jetta to drip water on the passenger side floor board?

Manufacturers took condensation into account when they designed your AC system.

The water you see is supposed to exit the system through a drain tube usually located about half way up the passengers side firewall and hangs down, it is the source of the little puddle of water you sometimes see under your car after the AC has been on.

When the tube becomes plugged the water from condensation will accumulate until it overflows into the passenger compartment.

To remedy this slide a piece of wire up the tube until the blockage is gone and water flows down the wire, you can do the same with some compressed air.