LogMeIn Hamachi
"Fakes" a LAN, over the Internet. Basicly, this means you can use LAN in games, even while playing across six timezones.
(https://secure.logmein.com/products/hamachi2/)
Rainmeter
Rainmeter is a desktop customization platform. Through Rainmeter, you can enhance your Windows computer at home or work with skins: handy, compact applets which float freely on your desktop, much like Windows Sidebar gagdgets, or dashboard widgets for the Mac. (Takes forever to setup, but it's WICKED)
(http://rainmeter.net/RainCMS/)
Skype
Your basic Voice-over-IP program, which kicks the ass of MSN, XFire and AIM (IMO)
This will even let you make actual phone calls, cross-countries, for a much lower fee than your phone company, which I find is very useful.
(http://www.skype.com)
VideoLan VLC Media Player
This thing plays everything. EVERYTHING. It's simple, doesn't take up loads of screen space with half a million menus and lists, and generally just pwns the living £@&e¤ out of any other I've used in the past.
(http://www.videolan.org/vlc/)
Audacity
One of the better freeware sound editors available. Lets you edit pretty much all aspects of a music track, and has several optional extras for working with different encodings and so on.
Personally use this a lot to highten the sound level of music tracks, to match that of my current library. (So I don't have to fiddle with the sound control all the time >.<)
(http://audacity.sourceforge.net/)