If you are interested in libertarianism, free markets and classical liberalism, these links might prove helpful. I've made no attempt at completeness. Please don't be offended if your organization or site is not included. My goal is to get interested parties some more information.
Note: Boldfaced links are ones I highly recommend based on my personal preferences.
Cato Institute is the premiere Washington-based libertarian think tank. Publishes journals, magazines, books, and papers. Also, lobbies Congress and sponsors conferences.
Civil Society Institute is a Santa Clara University libertarian group that promotes discussion of public policy.
Critical Review is a peer reviewed journal featuring discussions of libertarianism, free market economics, social theory, and social philosophy.
Free-Market.Net is a libertarian portal. Lots of links to places libertarian on the web.
The Independent Institute is a libertarian think tank. Publishes The Independent Review and books appyling libertarian ideas to life and society.
Institute for Economic Affairs is Britain's original free-market think-tank, founded in 1955.
Istituto Bruno Leoni is one of Italy's free market think tanks. (The site is in Italian.)
The Jefferson School's primary mission is further development, application, and teaching reason, individualism, capitalism, and of compatible ideas in the field of psychology. Basically a Objectivist-libertarian center.
Laissez Faire Books retails of libertarian books. Lots of works at huge discounts. Also, promotes online discussions.
Libertarian Alliance Based in England, this organization has lots of publications (many available online) and carries out many educational activities.
Liberty Fund funds conferences, research, and sells classic books on freedom – works by Smith, von Mises, Hayek – scholarship at cost.
Liberty Magazine is the libertarian's libertarian magazine. Not for the neophyte. Often features internal squabbles. A good watering hole for various factions. Available on newstands in many areas of the U.S.
Ludwig von Mises Institute is a research and educational center for classical liberalism and "Austrian" economics. Strong focus on political economy.
Molinari Institute takes its name from Gustave de Molinari (1819-1912). He originated of the economics of market anarchism and is a major inspiration to libertarian anarchists to this day..
Mont Pelerin Society was founded in 1947 and analyzes social and cultural problems from a classical liberal perspective. Hayek was a founding member.
Reason Foundation Promotes rationality and freedom. Also publishes Reason, a major libertarian magazine available on many newstands in the U.S.
The Smith Center for Private Enterprise Studies sponsors lectures, research, seminars, conferences, and courses on free market political economy.
Society for the Development of Austrian Economics promotes "Austrian" economics. Many prominent economists are members, such are Roger Garrison, Stephen Horwitz, and David Prychitko.