Three main possibilities:
1. There is a leak somewhere which is allowing water from outside to get inside the car. On a hardtop, the windscreen glass and the area around the rear bumper attacment points are the two most likely areas to check, but on a ragtop, there are many more possibilities. The key feature in all of these is that it is more noticeable after rain.
2. The A/C drain is blocked, allowing condensate from the air conditioning unit to collect in the cabin.
3. A pipe has split in the windscreen washer circuit, most likely the one to the rear screen.