too many clients

What does this mean and how do I diagnose it?

java.sql.SQLException: FATAL:  Sorry, too many clients already

Not all the Postgres databases on this machine are experiencing this.
One of them is using pooling with Tomcat and the JNDI interface.

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____________________________________________________________
Glenn Holmer                          gholmer@weycogroup.com
Programmer/Analyst                       phone: 414.908.1809
Weyco Group, Inc.                          fax: 414.908.1601