The Jedi Visual Component Library (JVCL) is a library of over 300 open-source components for CodeGear's Delphi product. It is compatible with Delphi 6.0, and higher versions, including the new Delphi 2009.
The homepage for the JEDI VCL is here:
The JEDI VCL contains only components, but it requires the Jedi Code Library (JCL) as it uses many JCL units. Thus you must install the JCL before installing the JVCL, and you cannot use a brand-new JVCL with old JCL versions. Even if you have the JCL already on your computer, it's a good idea to download a ZIP file from the Sourceforge project page, containing both the JCL and JVCL in one ZIP file, to be sure you don't mix/match the wrong JCL version with your JVCL.
The JCL and JVCL have separate installer routines, launched by running the install.bat batch file. The command line delphi compiler is used to build and install your projects. You can not install this package while the Delphi IDE is running on your computer. Shut down Delphi, install this, then start up delphi again.
For more about the JEDI project and it's various sub-projects, see the main JEDI wiki page.