Sticky #1: Skype (FMOI)

It is a VOIP telephone service, it's free and good for international calls. It's a way to make inexpensive to free calls using your PC as the bridge.

Quality is better than regular phone (no delays), provided the PC's at both ends have suitably-set up audio.

Conference feature is also useful.

Skype is only free when you're calling another Skype PC. For $.017 Euro's a minute, it is a fabulous way to talk to the world cheaply. If you're calling someone who doesn't use Skype, you'll pay these rates (in euros):
http://www.skype.com/products/skypeout/rates/

You can SkypeOut to call people not on a VOIP connection:

If this was a perfect world, there’d be no yodelling, we wouldn’t be ashamed of our love handles and everyone would be using Skype to talk for free.

Unfortunately the world is far from perfect, and not everyone has Skype yet. That’s why we have SkypeOut, a low cost way to make calls from Skype to friends who still use those traditional landlines or mobile phones. That means calling anyone, anywhere in the world at local rates.

Just as an example, when you use SkypeOut to call the United Kingdom you can save up to 80% per minute compared with Bell Canada.

The application doesn't come with any spyware and runs seamless on the PC machine. It doesn't crash and it has become the de facto mean of communication in many companies with offices all over the world. The conference call feature is very cool. There is an OSX version that will come out which I'm sure it will be defined as a killer app.