Libertarian Alliance - broad, UK-based libertarian pressure group.
Adam Smith Institute - Free market economics think tank, named in honour of the famous British economist.
Institute For Economic Affairs - Another British economics institute. Highly recommended.
Cato Institute - respected Washington D.C. based policy institute.
Ludwig von Mises Institute - hardline, radical libertarian economics institute, named in honour of the free market economist.
The Objectivist Center - Probably the better of the two US institutions promoting the Objectivist philosophy of Ayn Rand, which has profoundly influenced the modern libertarian movement. The other is the Ayn Rand Institute, who would not wish to be associated with a "libertarian" group and who make a habit of expelling talented individuals (such as David Kelley, who then founded the Objectivist Center).
The Jefferson School Of Philosophy, Economics and Psychology - run by George Reisman and Edith Packer. Two more of the "talented individuals" referred to above.
The Free Radical - terrific New Zealand-based libertarian publication, run by independent minded (i.e. non-ARI-affiliated) Objectivist Lindsay Perigo. The site also contains a section on the singer Mario Lanza.
Sense Of Life Objectivists - up and coming Objectivist group associated with the Free Radical, and placing considerable emphasis on esthetic and cultural issues as well as politics and philosophy. By far preferable to either of the large institutions.
Acton Institute - exploring liberty from a religious (especially Christian) perspective.
Lew Rockwell - Roman Catholic, cultural conservative, anarcho-libertarian :-)
Movement Of The Libertarian Left - Otherwise known as Agorism. Associated with the late, great Samuel Edward Konkin III.
Agorist Institute - More of the above. Not entirely to my own taste but radical and thought-provoking stuff nonetheless..
St Andrew's Liberty Club - Libertarian student group at St Andrew's University, Scotland.
LSE Hayek Society - Libertarian group at the London School of Economics.
Chris' Libertariansim Blog - This guy sent us an email asking if we'd add his blog to our links :-)