I used to love Visio before Microsoft acquired it. I never liked the direction they took with the tool, and I liked each newer version less. I also do not know if it still exists, I have not heard anything about it in a long time.

CA ERwin (www.erwin.com) is arguably one of the best modeling tools. Probably also one of the most expensive. I used the (free) community edition when I needed a way to create ER diagrams for a data modeling course, but the restrictions on number of objects and functionality of that edition mean that you probably will not be able to use if for diagramming a real database. If you can get a budget, my recommendation is to first play around with the community edition, do the same in free/preview versions of other modeling tools to see which you like best, then ask your boss to buy a licence for you.