I've been using a Hoodman Eyecup on my 5D Mark II and 40D for a good while now and I really like them. They are ambidextrous so you can turn it 180 degrees to suit you eye positioning. I like it a lot because it blocks out all of the periferal light that can enter around the eye and eye cup. The only two draw-backs: 1. Part of the eye cup blocks some of the LCD screen (not much and you get used to it). 2. Probably won't work for eyeglass people.
Here's the different version on Amazon:
Hoodman Eyecup