The most interesting tool I found is from HGTV's Bathroom Design Center, which allows you to take some pre-made bathrooms and adjust them to your liking. I found it to be very useful to match colors for your bathroom. It helps most matching up colors for your faucets and countertops or your sinks and bathtubs.
Some downsides to the Design Center are the choppy graphics. You can't make out all the details in the products. Also sometimes it's difficult for it to run properly on your computer. In addition, you can only choose Kohler products. So if you aren't a fan of Kohler then you won't be pleased with this tool. I couldn't change the tub faucet on the tub either, which bummed me out a little.
Kohler also offers a bathroom design tool on their website. Kohler's program is more in-depth than HGTV's. Allowing you to customize your room size, and put add whatever Kohler products they have on their website into your bathroom.
Here are the products I used in the bathroom designs featured above.