Download and install FREE Qt Open Source versions, on Ubuntu 13.04.

Qt can be used under open source (LGPL v2.1) or commercial terms. If you are Enterprise and want to try Qt, visit http://qt.digia.com/Try-Qt-Now/ to TRY the commercial version. Alternatively, you can download the Open Source versions by visiting http://qt-project.org/.

Visit http://qt-project.org/, click the Download link.

Download Qt Open Source versions 

Scroll down to download the file for your system. In my case of Ubuntu 13.04, download Qt online installer for Linux 32-bit.

download Qt online installer for Linux 32-bit

Open File Manager, browse to the download file, qt-linux-opensource-1.4.0-x86-online.run. Right click it and check allow the installer to runs as a program.

Allow the installer to runs as a program

Open a Terminal, change to the folder of the installer. run it by enter the command:

$./qt-linux-opensource-1.4.0-x86-online.run

The extra steps is quite straightforward:

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Remark:
(for Ubuntu 13.04)

If you reported with the following errors:

error: Qt Creator needs a compiler set up to build. Configure a compiler in the kit options.
Visit Install g++ on Ubuntu

error: cannot find -lGL.
Visit Install OpenGL library