Wikipedia article on the Horn - general characteristics, history, types of horns, repertoire, horn players, photos of horns.
Horn History - Brigitte's Horn Page offers a good concise history of the horn in four sections: 1. Ancient brass history up to 1600 2. The hunting horn in France from 1600 to 1700 3. Crooks and hand horn technique 4. The modern valve horn. 115 footnotes!
Dick Martz – Horn Collection
Dick Martz collections historical horns, and displays beautiful photos of his remarkable collection online.
History of the Horn - a collection of links, part of "Horn Player's 'Where to Find'"
The Hunting Horn - concise information on the instrument, history, technique, and repertoire by the Koninklijke Antwerpse JachthoornKring
A Brief History of Horn Evolution - part of Tom Bacon's Cyberhorn Museum (check out the rest of the site!)
Instruments of Mass Seduction II - Article on horn history, repertoire, and players by Frederick Kirshnit on Concerto.net.
A Brief History of Brass Instruments
A good short history by trombonist David Wilken of the U of North Carolina.
National Music Museum
The NMM is located in Vermillion, South Dakota, and has a stunning list of collections of every kind of musical instrument, including horns.
The Horn in Bach's Vocal Works
The Solo Works for Horn of Franz Strauss
A timeline by John Ericson, from a presentation at the 35th IHS workshop.
Joseph Meifred and the Early Valved Horn in France - by John Ericson (this is a name that is going to come up a lot in links to historical articles on the horn...)
The Natural Horn Today - excellent article by Jeff Snedeker.
Article by Nicholas Smith.
Russian Horn Capella - the unique Russian Horn Capella [band], where each instrument can only play one note, has been revived in Russia!
The Brass Players Museum - Antique brass instruments museum in Springfield, MA.
Historic Instruments Worldwide - Tom Bacon's list of historic instrument collections
The Persistence of the Natural Horn in the Romantic Era - by Ken Moore. Thoughtful, well-documented study of the use of the natural in the later 19th century.
Elias Lewy and the First Performance of Beethoven's Ninth Symphony - article by Theodore Albrecht in the web site of the International Horn Society
The Natural Horn in France - new web site, mainly in French (with some English) by Claude Maury. Loads of information: recordings, horn players, links, discussion group, blog.
The 19th Century Horn - a series of articles by the indefatigable John Ericson.
Berlioz on the Valved Horn - by John Ericson (excerpt from his treatise on orchestration)
Henri Kling and the Valved Horn in the Late 19th Century - John Ericson again
Oscar Franz and Richard Strauss on the Horn in the Late 19th Century - Ericson, John
A Bibliography of Writings about Historic Brass Instruments - compiled by David Lasocki, music librarian at Indiana University (and alumnus of The University of Iowa!)
On the Skill of the Viennese Brass Instrument Makers - article (pdf) by Jeremy Montagu
The omnitonic
horn: read articles on it by John Ericson and Dick Martz.
Early Valve Design - by John Ericson
Early Horn Mouthpieces by Richard Seraphinoff
Selected Horn History Resources - bibliography by John Ericson