现在介绍几个相对比较好用的工具：
第一个：ERDesigner NG
官方网址是：http://mogwai.sourceforge.net/?Welcome:ERDesigner_NG
属于sourceforge的一个开源产品，目前版本为1.4
以下是官方所描述的：
程序代码
The Mogwai ERDesigner is a entity relation modeling tool such as ERWin and co. The only difference is that it is Open Source and does not cost anything. It was designed to make database modeling as easy as it can be and to support the developer in the whole development process, from database design to schema and code generation. This tool was also designed to support a flexible plug in architecture, to extend the system simply by installing a new plug in. This way, everybody can implement new features and tools to make ERDesigner fit the requirements.
官方网址是：http://mogwai.sourceforge.net/?Welcome:ERDesigner_NG
属于sourceforge的一个开源产品，目前版本为1.4
以下是官方所描述的：
程序代码
The Mogwai ERDesigner is a entity relation modeling tool such as ERWin and co. The only difference is that it is Open Source and does not cost anything. It was designed to make database modeling as easy as it can be and to support the developer in the whole development process, from database design to schema and code generation. This tool was also designed to support a flexible plug in architecture, to extend the system simply by installing a new plug in. This way, everybody can implement new features and tools to make ERDesigner fit the requirements.
ERDesigner NG
* is based on Java and can be run on Windows and Unix systems
* has a powerfull WYSIWYG for physical database design
* handles tables, relations, indexes and comments
* supports subject areas
* supports MySQL, oracle, Microsoft SQLServer and Postgres
* creates the SQL DDL statements for schema creation
* has an integrated schema version control system
* can generate schema migration scripts for every change
* stores the database definition as XML files for further processing
* can export the database schema as GIF, BMP, JPEG or SVG files
* has an integrated reverse engineering module for existing schemas
* it is based on GPL license
* support is available by authors and newsgroups
从上述的描述我们可以看得出，软件支持多种主流的数据库，比如mysql、oracle、MSSQLSERVER等。
功能方面支持反向工程、数据库比较、通过建模自动产生ddl、将模型图导成图片等等。
软件的截图如下：
第二个：ModelRight3
官方网址为：http://www.modelright.com/Default.aspx
以下是官方的介绍：
程序代码
Need to design, visualize or document your database?
ModelRight 3 can reverse engineer your existing database to give you a graphical view of it, generate all the SQL statements to create the database, or synchronize your database model with the database to keep your design up to date.
Do you need to control the most detailed aspects of your database?
ModelRight 3 has more complete and in-depth support for your database's (Oracle, SQL Server, MySQL or any via ODBC) most advanced features.
别以为真的支持这么多数据库，这个是专业版的才知识这么多，如果你叫钱多多，那你就去买专业版吧！
这个产品是比ERDesigner NG专业的一个，理由非常简单，它本身就是一个商业的工具，不过，也有免费版本，只是免费版权在数据库的支持上少了好多！今天我们介绍的就是这个免费版本，只支持mysql，比较痛苦！产商其实不太厚道！
下个图片截图吧！
再来个官方的图片（专业版的）
看了专业版的千万不要眼红，使用是要付出代价的！如果我们每个人都是钱多多，多好啊！
第三，OpenSystemArchitect（推荐产品）
官方网址是：http://www.codebydesign.com/
这是一个开源做得比较彻底的一个产品，推荐大家使用。
最喜欢的一点说法是操作习惯有点类似于PowerDesigner，界面比PD丑一点，但是，还挺好用的！
它支持windows、Linux
以下是官方的介绍：
程序代码
Open System Architect (OSA) is an application used for modelling a system. OSA currently supports data modelling (physical and logical) with UML in the works. The unique thing about OSA is that it is an Open Source product licensed under the General Public License (GPL). The free factor makes OSA particularly interesting for students but OSA is powerful enough to compare favourably with products costing thousands of dollars.
OSA is sponsored by CodeByDesign (the originally developer of OSA) and by Source Forge (they host the community collaboration environment for OSA).
以下是官方的demo，看完这些截图之后，我相信你马上会下载一个用用！ 为什么，确实是厚道！
Logical Model
Physical Model
SQL Editor
第四个：Mysql WorkBench
官方网址：http://dev.mysql.com/workbench/
Mysql官方出的设计工具，for mysql
如果你是使用mysql，也可以考虑一下它！