Came across a link to several articles at wisegeek.com about Punctal Plugs (Tear Duct Plugs):
what are punctal plugs
Also on wisegeek.com, there is an article about the Schirmer Test - a test that helps doctors diagnose various medical conditions that cause dry eyes:
what is the schirmer test
The website DryEyeSyndrome.com has excellent resources, which includes explaining the various types of plugs:
encyclopedia plugs
DryEyeSyndrome.com also has excellent forums for those suffering from dry eyes and those who want to discuss the plugs:
forum PLUGS-etc