That is very common.  The best way to prevent it is to keep the lid
down.  

                 ---MIKE---

Several possiblities exist, as I see it:
1) Kitty is a dumb sh*t (No, cats are notoriously smart and have a
reason for everything they do. It's up to us to figure it out.)
2) Kitty is thirsty (Not likely, especially if s/he he has an
alternative water source.)
3) Kitty is curious ( Probably not.  More likely, s/he has come to the
great white throne for refreshment on numerous occasions, and this is
the first time s/he has been caught in the act.)
4) Kitty is showing you how "cool" s/he is (Did she do this for
attention?)
5) Kitty is trying to annoy and disgust you (Bingo - this is the most
likely scenario.  The next maneuver is to hop up on the counter and
chew on your toothbrush and then flash you that winning smile!)

Moral of the story:
Keep the lid closed (and your toothbrush in a cabinet or drawer).

Seriously, your cat might prefer water that has been standing in the
open air for a while.  This allows the chlorine that is used to treat
domestic water supplies to dissipate, so the water smells and tastes
better.  It's silly, but I bought my cat one of those water-cooler
type water dishes and fill it with water from my Britta pitcher.
Since I started doing that, she hasn't shown much interest in toilet
drinking.  Also, I do try to keep the lids closed, especially if I use
those in-tank cleaning tablets.  (Not so good for animals.)

Cheers,
Gary

I bought a pet mate water fountain...

My cat stopped the toilet drinking after that... they are cheep at wal-mart...