One of the best tools out there, regardless of cost, is EasyCASE from Evergreen CASE Tools
Evergreen CASE Tools:
EasyCASE (shareware); EasyCASE plus (commercial)
11025 164th Ave NE; Redmond WA 98052; 206-881-5149
I've used many different tools in the course of my 22 years in this industry, and EasyCASE is not only simple to use and extremely powerful, it is also remarkably inexpensive. IEW/ADW from Knowledgeware are great, too, but too pricey for the individual developer; and Texas Instruments' IEF is the pinnacle of whole-system development tools, but it is prohibitively expensive for all but the largest of corporate IT budgets.
Overall, EasyCASE is the hands-down winner in terms of price/performance when it comes to data-modeling tools.