As many other things, ERD tools can also be found on the Internet using one of search engines.
Different people use different tools; it also depends on your budget - there are free (even open-source) products and, on the other hand, expensive ones. Furthermore, your database might help - use DBDesigner for MySQL, Oracle Designer for Oracle, etc.
Or, you could try ERWin, one of the market leaders, for example.
Can anyone suggest a good software for drawing entity relationship diagrams?
Have a look at this. http://www.databaseanswers.com/modelling_tools.htm
It's a great site about dbs anyway.
I also use DBDesigner to work with MySQL, if you get version 5 you 'll find a more sophisticated solution of it, although still in beta version and a bit buggy.