Xobni

This Outlook add-on has been a big hit in the office, and as soon as you see it running you'll understand why. Xobni scours your emails to generate what its creators call an "automatic address book", showing the conversation threads and attachments you've shared with each person you've had contact with, and statistics for how often you email them.
Xobni goes as far as actually building a mini profile of each person, including phone numbers and contacts details and Facebook accounts where possible (yes, this includes photos - one interesting aspect of using Xobni is you may suddenly see the face of people you've only ever known through email).
Plus you can see all your conversations with that person from one window, as you do on your smartphone, without searching for emails. The end result is a much quicker way to follow conversations, check someone's contact details, and rely less on maintaining Outlook's contact list. The free version is limited to one pst file.
LAME

If you're serious about building a digital music library, you don't want to use iTunes to rip the music from your CDs, and you're using free software to do it, you'll be wanting this free encoder. It's used or supported by many well known free audio programs, including the excellent Audacity editor, which we included in our list of the 99 Best Free Apps in the November 2009 issue of PC Authority. It's not a program as such, but LAME makes our list as a must-have tool for building a quality MP3 library.
Skype

First released as a beta in 2003, Skype has become the most well-known consumer VoIP program in the world, and is even more useful since the software became available on smartphones.
The big news (at the time of writing) is the possibility of a version that allows Skype calls over 3G on the iPhone, instead of only via Wi-Fi. Skype is also being built into TV sets, and there's a new group video chat feature that was in beta at time of writing. A must-have if you have long distance friends or relatives.
Malwarebytes

There are a number of highly regarded free security programs, including Threatfire, which made our list of the 99 Best Free Apps in the November 2009 issue of PC Authority, and Microsoft Security Essentials in this month's list. If you want an extra tool up your sleeve, Malwarebytes Anti-Malware is our pick.
While some might be tempted to run it alongside a security suite, we'd recommend against doing that. Instead use it as an on-demand scanner for when your main protection fails, or as a resource-light tool for de-infecting a friend's system. The free version limits you to manual scans.
Souptoys Toybox

Looking for a way to keep the kids entertained on your computer? SoupToys is the sort of kids' game that we just haven't been able to stop playing with since we downloaded it. The game's makers describe it as a "physics sandbox" game for kids five to 12 years old - players can build "playsets" using balls, bees, pirate ships and other objects.
The best bit is being able to share your playsets online - there are apparently more than 20,000 to download, with names like Egg Launcher: "just keep making the goose lay eggs which will go into the cannon which will attack the dinosaur".