10 Best free and open source

These days, you can quite easily buy a brand-spanking-new computer and install all the software you need for free, using applications offered under the GNU/Open Software License. 
You can get a free operating system, image editor, a free sound editor, office suite, media player, file archiver, PDF creator… the list goes on and on and on. The vast majority of them are cross-platform. 
Listed below are 20 invaluable and indispensable open source applications that you really should be using, if you’re not already.

Mail Client: Thunderbird
With its speedy searches, strong security and superb add-ons, Thunderbird has to be the best, free email application available. If you’re prepared to spend some time tailoring your email environment with add-ons, you’ll absolutely love it. Outlook users will feel right at home. Addons available from customization, themes, and importing VCF's (contact cards)

Photo editor: GIMP
As for most Photoshop users understand, image editing software is expensive for most people, but GIMP provides a free alternative. It performs every major function you could desire, including working with layers, image retouching and editing, transform, resizing, cropping. File formats include GIF, JPEG, TIFF, PNG, PSD, BMP...etc

Office Suite: Open Office
With the ability to create text documents, spreadsheets, presentations and databases, OpenOffice is an accomplished rival to Microsoft Office, which clearly influenced OpenOffice’s design. Microsoft Office users will feel completely at home and find that OpenOffice performs just as well, if not better.

Operating system: Ubuntu or Linux Mint
Ubuntu is a free operating system that’s quick and easy to use. Recent figures suggest that around 50% of Linux users have Ubuntu installed and was, until recently the number 1 choice for Linux ditros. Linux Mint has taken the top lead. Mint is actually based on Ubuntu but it adds a custom desktop and menus, and avoids the new Unity desktop which Ubuntu utilizes.

Accounting/Small Business: GnuCash
GnuCash provides a great, free alternative to paid-for accounting software. Designed for personal and small business use, it offers bank account, stock, income and expense tracking, in addition to double-entry accounting.

Music/sound editor: Audacity
Users can record and edit live audio; cut, copy, splice and mix sounds; and convert ageing tapes and records into digital format. Formats supported: MP3, WAV, MID, AIFF,
and Ogg

Zip Compression: ZipCentral
ZipCentral is a free and easy to use ZIP file manager with all the utilities you need to manage your zip files. It can also create Self-Extracting archives and archives spanned across multiple files. ZipCentral supports Drag & Drop to and from Windows Explorer Context menu.

FTP: FileZilla
FileZilla is a hugely successful, cross-platform FTP client. It’s also available as a server, for Windows only. Created in January 2001 by Tim Klosse as a class project, FileZilla has gone on to become the 5th most popular download of all time from SourceForge.net.

PDF: PDFCreator
Windows users can consider PDFCreator a credible rival to Adobe Acrobat. Creating PDFs is as easy as printing, literally, as once installed, you can select PDFCreator as your printer, letting you create PDFs from practically any application.

Blogging/Website: Wordpress
WordPress is the world’s most popular blogging platform, used by a staggering 202 million websites. As simple or as complex as you want it to be, WordPress is supported by a wide array of plugins which can be used to transform a standard blog into anything you could possibly desire.