Here is the list of the must have free apps for Linux
1. Konqueror - Konqueror is a web browser, file manager and file viewer designed as a core part of the K Desktop Environment. It is developed by volunteers and can run on most Unix-like operating systems. Konqueror, along with the rest of the components in the KDEBase package, is licensed and distributed under the GNU General Public License.
2. Lynx - Lynx is one of the most popular web browsers for command-line interfaces. Lynx runs on Un*x, VMS, Windows 95/98/NT, DOS386+ but not 3.1, 3.11, or OS/2 EMX.
3. Thunderbird - Thunderbird is a good quality graphical email client that uses the GTK toolkit but is not tied to any particular desktop environment. It has pretty much all the features you would expect to find in such a program: mailing list handling, encryption and digital signatures for outgoing and incoming mails and plenty of filtering options.
4. KMail is the email component of Kontact, the integrated personal information manager of KDE.
5. Evolution - Evolution is the most feature-rich of the mailers we looked at – at least as they were set up out of the box – largely because of the comprehensive range of plugins provided, although the one to play audio attachments could get a little tiresome, especially if deployed in a large office.
6. OpenOffice.org - OpenOffice.org (OO.o or OOo) is a cross-platform office application suite available for a number of different computer operating systems. It supports the ISO standard OpenDocument Format (ODF) for data interchange as its default file format, as well as Microsoft Office ‘97–2003 formats, Microsoft Office 2007 format among others.
7. KOffice - KOffice is an office suite for the K Desktop Environment (KDE). All its components are released under free software/open source licenses. KOffice applications use OpenDocument as their native file format when possible and the suite is released separately from KDE and can be downloaded at the KOffice homepage.
8. MPlayer - MPlayer is a free and open source media player distributed under the GNU General Public License. The program is available for all major operating systems, including Linux and other Unix-like systems, Microsoft Windows and Mac OS X. Versions for OS/2, Syllable, AmigaOS and MorphOS are also available. The Windows versions works, with some minor problems, also in DOS using HX DOS Extender. A port for DOS using DJGPP is also available.
9. Totem - Totem is the official movie player of the GNOME desktop environment based on xine-lib or GStreamer. It features a playlist, a full-screen mode, seek and volume controls, as well as keyboard navigation.
10. Gedit - Gedit is a free software, UTF-8 compatible text editor for the GNOME desktop environment. It is mainly designed for editing program code, and structured text, such as markup languages. It is designed to have a clean, simple graphical user interface according to the philosophy of the GNOME project.
11. KPhotoAlbum - KPhotoAlbum (previously known as KimDaBa) is an image viewer and organizer for Unix-like systems created and maintained by Jesper K. Pedersen. The core philosophy behind its creation was that it should be easy for users to annotate images and videos taken with a digital camera. Users can search for images based on those annotations (also called categories) and use the results in a variety of ways. Features include slideshows, annotation, KIPI plugin support for manipulating images, and boolean searches.
12. DigiKam - DigiKam is an image organizer and editor for the KDE desktop environment. It supports all major image formats, and can organize collections of photographs in directory-based albums, or dynamic albums by date, timeline, or by tags. Users can also add captions and ratings to their images, search through them and save searches as ’smart folders’. With the plugins they can also export albums to Flickr, Gallery2, Google Earth’s KML files, Simpleviewer, burn them on CD, or create web galleries.
13. GCompris - GCompris is a suite of educational software for children aged 2 to 10. It is available for Linux, Mac OS X and other systems. A Windows version is available as crippleware with a restricted number of activities; it is possible to access all the activities in Windows for a fee.