Row Perfect - http://www.row-perfect.nl/ - causes ~6x less load for knees than Concept2. There is also research behind the technology and therefore has much more professional approach than others.
Concept2 - http://www.concept2.com/
I was member of Estonian national rowing team 1990-1995.
Also in 1985-1995 I was active athlete of academic rowing in Pärnu, Estonia