Here are free software that you should try using tme if you are not already.
Operating Systems (OS)
Ubuntu (Free user friendly Linux Operating System for the desktop + it comes with almost all the applications you may need such as word processors, media players, image editors, text editors, CD burning tools and load of other applications)
Ubuntu Server Edition (Free and fast powerful enterprise server)
Windows ($$ This is not free Microsoft Windows has discounted priced Operating Systems and Applications for schools)
Anti-Virus Software
AVG (Provides free Anti-Virus edition) -Linux and Windows versions available
Avast! (Provides free Anti-Virus edition as well)
Microsoft Security Essentials (This is free Windows security tools from Microsoft. This enough for protecting your home windows based PC. Remember to update frequently)
Spybot-S&D (free Spybot Search & Destroy is one of the best anti-spyware software available today.)
Encryption
T r u e C r y p t (Free disk encryption software for Linux, Windows and Mac OS X)
Internet Browsers
Firefox (Mozila Firefox is one of the best free internet browsers available)
Chrome (Chrome is free internet browser from Google)
Internet Explorer ($$ comes with Windows operating Systems which is not free)
Document Processors
OpenOffice (free word processing applications similar to Word, Excel and PowerPoint)
Notepad++ (Free text editor similar to notepad)
7-Zip (Free File Archive and compression tool)
Sumatra (Windows Only) free, open-source PDF reader for Windows.
Email Systems
Local : Thunderbird is a free email system offering intelligent spam filters, built-in RSS reader, quick search, and more. It is a local (rather than a web-based) email application that is powerful yet easy-to-use.
Web based : Gmail (free web based email account accessible from any computer with internet connection)
Yahoo! (free unlimited web based email account accessible from any computer with internet connection)
Instant Messengers
Pidgin (Free (Linux, Windows, Unix) chat application that lets you chat with all your friends on AIM, MSN, Yahoo, ICQ, Google Talk, and many others)
Media Players
VLC (free multimedia framework, player and server)
Image Editors
Paint.NET (Free image and photo editing software for computers that run Windows. A good alternative to Photoshop.)
Gimp Cross platform graphic software for photo retouching, image composition and image authoring.
Graphic Converter Graphics software that allows users to open, view, resize, and edit in multiple images formats.
Downloading/Uploading Stuff
Free Download Manager: The FDM accelerates downloads by splitting files into sections and then downloading them simultaneously. As a result download speed increases many folds. If you lose connection you can continue where you left off saving valuable time and money for people with low quality connectivity and expensive internet such as common in Africa.
BitTorrent (peer-to-peer)
µTorrent (Linux, Windows, Mac) A free tiny BitTorrent for peer-to-peer download and upload.
FileZilla (free FTP software solution software (Client & Server) is available.)
Software Utilities
Gparted (A free graphical partition editor for creating, reorganizing and deleting disk partitions)
VirtualBox (VirtualBox lets you create virtual machines for enterprise as well as home use)
Education Management Software
Content Management Software: Drupal is an open source content management platform powering millions of websites and applications. It’s built, used, and supported by an active and diverse community of people around the world. Use Drupal to build everything from personal blogs to enterprise applications. Thousands of add-on modules and designs let you build any site you can imagine.
Library Management: Koha Library circulation, cataloging, and acquisitions system
Course Management System: Moodle Course Manegement System (CMS), it is a free web application that educators can use to create effective online courses.