I just bought a Kurt Kinetic. It is a magnetically-coupled fluid system - the magnet is the coupler between the roller and the impeller, so there is no penetration of the fluid-holding cavity. Supposedly, no leaks. The only other kind of trainer I am familiar with is the fan or air trainer - they are quite loud.

We tried the Cylceops and the Kurt trainers - the Kurt was louder, but more stable and has the ability for you to increase the flywheel weight.