I was reading up on database design esp. 6NF and came across this website on Anchor Modeling. It has a HTML5-based Online Modeling Tool that looks nice and fun to play with.
Looks nice, but the constant floating around and 'wiggling' really bugged me. It took me quite a while to figure out how to add and remove things.
It also reminded me of an ORM tool which I helped build in college called nORMa. It was a plugin for Visual Studio and was great for designing complex databases.