We are using Google SketchUp. It's free but a bit cumbersome to learn-- actually using it to do the entire remodelling project. You can get very precise dimentions. We played with this to figure out layout options but will use the complementary kitchen design service at our local lumber yard before finally deciding.

Homestyler.com is another free online tool that works pretty well but it's limited.

Some companies use 20/20. I don't know if you can free download it or have a tryout.

+1 for Google Sketchup. I'm just a DIYer and use it to visualize and communicate what I want with GCs and Vendors.

Like everything though, there is a learning curve. I've found there are tons of tutorial videos on YouTube though. Just google 'Sketchup YouTube'.