If you like me use Skype quite bit for calls or coding together with other developers, you might find that when you're quiet Skype tries to be clever and automatically increase the microphone gain. This is crap as it starts to pick up the background noise.
You can't easily turn off automatic gain control in Skype, it's a setting you can apply in a xml file. Hunting around I found the solution again and this time I've stuck in a gist in case you want to try it too. Try it, even if you have a headset with a microphone built in, this makes a difference.