Open Source Anti-Spam

ASSP (short for "Anti-Spam SMTP Proxy") works with existing Exchange, Lotus Notes, or other SMTP servers to stop the delivery of spam. Note that this a product for organizations, not home users, so you will need to be fairly knowledgeable to use it. Unlike the commercial Barracude product, it's software, not a physical appliance, and it only stops spam, not viruses. Operating System: OS Independent.

Open Source Anti-Virus/Anti-Malware

One of the best-known open source security software, ClamAv's anti-virus capabilities have earned it a stellar reputation. The version available at this site works on Linux/UNIX and is most suitable for larger organizations. Operating System: Linux.
Recently released, the official ClamAV for Windows is a partnership between SourceFire (the owner of ClamAV) and cloud-based security provider Immunet. Because it runs in the cloud, this anti-virus software doesn't drain system resources, and it's able to use community-based protection to improve security. And while both Panda Cloud Antivirus and ClamAV for Windows are free, only ClamAV has source code available. Operating System: Windows.
Suitable for home users, this front-end for the ClamAV antivirus offers good protection from viruses and integrates into Windows Explorer and Microsoft Outlook. Unlike the commercial products, however, ClamWin doesn't automatically scan files in real time as you download them—you'll have to schedule a scan of your system or manually click on a file to scan it individually. Operating System: Windows.

Open Source Anti-Spyware

Nixory works along with your existing firewall and anti-virus software to eliminate data-mining and spyware threats. While Spy Sweeper works with all browsers, Nixory is specifically designed for Firefox users. Operating System: OS Independent.

Open Source Application Firewall

Part of openSUSE and SUSE Linux Enterprise, Novell's application firewall uses policy-based profiles to control who can access various applications. This is not an appliance like the Barricuda product, but it does perform many of the same functions. Operating System: Linux.
ModSecurity provides a wide range of protection for Web applications. While this site offers software, not hardware, you can also purchase an appliance similar to the Barracuda product from Breach Security. Operating System: OS Independent.

Open Source Backup

8. Areca Backup Replaces NovaBackup
While not as robust as some commercial backup utilities, Areca Backup does make it easy to backup selected files and folders with compression and encryption. It's not a disk-ghosting application, which would allow you to restore your system completely, but it does save important files so you can find them again. Operating System: Windows, Linux.
Unlike Areca, Partp_w_picpath does save files in a way that provides easy recovery after a crash. It's also part of the SystemRescuecd package that contains a number of other helpful tools to help get your system back up and running. However, it's only available for Linux. Operating System: Linux.

Open Source Browser Add-Ons

Very similar to the SiteAdvisor, this add-on for Firefox, Internet Explorer, or Chrome rates sites on their trustworthiness. When you use one of the major search engines, WOT will show you a red, green, or yellow circle, so you know whether or not the site is safe to visit. Operating System: Windows, Linux, OS X.
PasswordMaker solves the problem of needing to create and remember multiple passwords. Although it works differently than the password managers listed above, it essentially solves the same problem. Operating System: Windows, Linux, OS X.

Open Source Data Removal

BleachBit erases the traces of your actions on your computer and speeds up performance by cleaning up your cache, deleting your history, and more. It can also "shred" files or folders so they can't be recovered from your hard drive. Operating System: Windows, Linux.

Even though you've deleted a file, someone may be able to recover it from your hard drive using forensics tools. However, Eraser writes over your old files, completely eliminating all traces of sensitive or personal files. Operating System: Windows
14. Wipe Replaces BCWipe Enterprise
Like Eraser, Wipe overwrites deleted files in order to make them impossible to restore. Operating System: Linux.
Sometimes you need to erase an entire hard drive—if you are recycling or donating an old system, for example. DBAN does the job for you quickly and easily. Operating System: OS Independent.

Open Source Encryption

The self-proclaimed "leading open source file encryption software for Windows," AxCrypt makes it easy to protect files and folders in Windows. From Window Explorer, you just right-click to encrypt, and double-click to decrypt data. Operating System: Windows.
17. Gnu Privacy Guard Replaces Cypherus.
Also known as "GPG," this Gnu app for encrypting e-mail and other communication is based on the OpenPGP standard—just like the PGP product. On the down side, this is a command-line project, so it's not particularly easy to use. Operating System: Windows, Linux, OS X.
This version of GPG was designed specifically for the Mac. Again, like the standard version, it's not particularly user-friendly, but it does get the job done. Operating System: OS X.
19. gpg4win Replaces Cypherus
And, as you probably guessed, this is a version of GPG for Windows. Unlike the other two versions of GPG listed here, this one includes extensive documentation, including a novice guide for beginners. Operating System: Windows.
20. PeaZip Replaces WinZip
Like WinZip, PeaZip's primary purpose is compressing files for archiving or transmission, but also like WinZip, it offers built-in encryption capabilities as well. PeaZip's interface isn't quite as easy to use as WinZip's, but it can create and read more file types. Operating System: Windows, Linux.
This file and folder encryption app was designed for maximum speed. Thanks to its small size, it works incredibly quickly—and you don't even have to install anything on your system in order to use it. Operating System: Windows.
Another file and folder encryption utility, NeoCrypt integrates with Windows Explorer much like AxCrypt. It's easy to use and supports ten different encryption algorithms. Operating System: Windows.
Like the PGP product, TrueCrypt can protect your entire hard drive or thumb drive. Incredibly popular, it's been downloaded more than 14 million times. Operating System: Windows.