Lazarus is a Rapid Application Development tool for Free Pascal and currently runs on Linux, Mac OS X, BSD and of course Windows.
It is freely available, open source and completely written in Free Pascal.

The official Lazarus site is https://www.lazarus-ide.org/.
There is a wiki with a lot of information around Lazarus at http://wiki.lazarus.freepascal.org/.
Free Pascal can be found at https://www.freepascal.org/.

The Lazarus Component Library is licensed under a modified GNU Lesser General Public License.
The Lazarus IDE is licensed under the GNU General Public License.

Free Pascal online documentation.
RTL - Free Pascal Run Time Library
FCL - Free Component Library
LCL - Lazarus Component Library
LazUtils - Lazarus Utilities Package

Many help files are available for download in chm and inf format.

Viewer for chm files: see Installing CHM help in the IDE
Viewer for inf files: see DocView IDE Integration.