GelSport in the US, it is a foam pad with a non-slip material sewn onto the bottom so all you have to do is place it on your rowing seat and go - no messing about with tie-downs or velcro or what-have-you.
Has anyone tried one? Are they any good? They are 25 bucks in the US. Here in the UK, distributor Stylo Sports only stock the Dragonboat version but apparently it fits nicely on the seat of a rowing machine so it should do the job. The price is £17 including P&P which isn't too bad.