Quit Skype and open the folloving file with TextEdit: ~/Library » Application Support » Skype » shared.xml. At the end of the document, you'll see this section:
<VoiceEng>
<MicVolume>77</MicVolume>
</VoiceEng>
Simply change this section to look like this (adding one new line):
<VoiceEng>
<AGC>0</AGC>
<MicVolume>100</MicVolume>
</VoiceEng>
AGC means Automatic Gain Control, and setting it to 0 disables this feature. Set it to 1 if you want to enable it again. The MicVolume can range from 0 (mute) to 255 (full volume). Save the file and start Skype.
Note: I'm using Tiger and Skype 2.6.0. I have no experience with other OS X and/or Skype versions, but I hope this hint will help some frustrated Skype users.
[robg adds: I haven't tested this one.]

