A. A. There are several on the market, including (in no particular order)
Also there are some tools built-in to Microsoft products. Note that these just do diagrams, whereas the 3rd party tools (with the exception of Visio) above have a wide-range of project lifecycle, reverse engineering etc. abilities.
Visual Database Tools does ER-diagrams
SQL 7.0 has built-in ER diagrams
MS-Access (you'll have to connect all the SQL tables first)