XCode is the new ProjectBuilder. To get the old Project Builder will require installing an older version of the tool. I can't remember which version or when Apple stopped distributing it.
The Xcode suite, like Mac OS X itself, has its roots in technology from NeXT. Prior to the October 24, 2003 release of Mac OS X v10.3, Apple referred to the suite of tools as "Developer Tools". The Xcode IDE was developed from and superseded Apple's earlier IDE, Project Builder, which was inherited and rewritten from NeXT's IDE of the same name.