Security (Here's a list of 21 open source security apps with commercial support.)
3) Anti-Virus (Linux): ClamAV
ClamAV is one of the world's best anti-virus scanners and forms the backbone of many commercial security products. If you have a Ubuntu-based notebook, you already have ClamAV in your universe repository, and most of the other desktop Linux distributions include ClamAV as well. To make sure ClamAV is running on Ubuntu, install the clam-avdaemon package.
4) Anti-Virus (Windows): ClamWin
To run ClamAV on Windows, you'll need to download and install ClamWin. This app will not automatically scan incoming files, but it does integrate with Windows Explorer and Outlook so that you can easily scan files manually.
5) Anti-Spam: SpamAssassin
The self-proclaimed "#1 open-source spam filter" has won numerous awards for outstanding capabilities in filtering out bulk e-mail. It comes in many flavors, and on the home page, you'll find links to a single-user installation for Linux/Unix users and a Windows installation.
6) Firewall: Firestarter
Many of the other open-source firewalls are suitable primarily for creating do-it-yourself network gateways and require a separate PC to use as an appliance. Firestarter can be installed directly on your laptop, and the GUI is simple enough for novices to use.
7) Password Protection: KeePass Password Safe
It really is dangerous to use the same password for all of your accounts, and it's even more dangerous to store all of your passwords unencrypted on your laptop. KeePass lets you store all of your passwords in an encrypted database that you can unlock with a single master password.
8) E-mail Client: Thunderbird
Made by the creators of Firefox, Mozilla's Thunderbird offers outstanding security, customization, and organization features. However, it does not include a calendar, which some Outlook users may miss.
9) Email Client: Zimbra
If you like having a calendar and to-do list integrated with your e-mail, Zimbra might be a better option. Zimbra also makes it easy to access all of your e-mail accounts in one place, and unlike web-based mail apps, it lets you access your e-mail even when you're offline. On the downside, it's still officially in beta, so you may find a few bugs.
10) IM: Pidgin
At last count, Pidgin lets you instant message with friends on 16 different networks, including AIM, GoogleTalk, MSN, and MySpace. It runs on both Linux and Windows, and has a number of add-ons available that extend its features.
11) File Transfer: FileZilla
Available for both Windows and Linux, the FileZilla client software lets you download ftp, ftps, and sftp files. It's easy to use and is available in a number of different languages.