Ricardo Matosinhos has crafted an online survey that lists type of player (professional/amateur/teacher), type of horn, mouthpiece model for a random and limited but interesting comparison of what players are using these days.
Colin Dorman has created a wonderful French Horn Mouthpiece Chart with over 500 cups, rims, and complete models, most with complete measurements. It also has a search field so that you can find a specific manufacturer. Colin says that it covers "all the makers I know about." His website also has a "Choosing a Mouthpiece" page.
Early Horn Mouthpieces - ditto
Holton Farkas (gold-plated)
The Farkas Model Mouthpiece - article by John Ericson on the most classical horn mouthpiece of them all.
The Wedge - Eric Reed Signature French Horn Mouthpiece ($185) - one piece mouthpiece designed for Eric Reed of the Canadian Brass. Double Oval Cup design that continues the oval shape of the rim into the cup and down into the throat of the mp. “Great for high horn, solo and chamber work, with a very middle of the road rim… it’s the most efficient mouthpiece on the market.” - Eric Reed. If you know which Moosewood or Laskey mouthpiece you like, you can order an analogous model.
Fluted Horn Mouthpiece - unusual new mouthpiece, apparently made by 3D printing… Read some commentary on it here. More commentary on Facebook.
Scott Laskey offers 4 sizes of mouthpiece diameters (17 to 18.5mm), 3 cup sizes, and a "unique rim contour". Superstar Gail Williams plays an 85GW Laskey horn mouthpiece. Pope Repair carries all Laskey models.
Moosewood - Custom horn mouthpieces, rims, underparts, + accessories (stem weights, visualizers, left hand supports, etc.). Maker Tom Greer also makes Vienna horn mouthpieces and will make custom mouthpieces to your specifications, and will repair damaged mouthpieces, and silver or goldplate a rim or mouthpiece.
Left to Right: Megamoose, Magnum Opus
Stork - Custom horn (and other brass) mouthpieces.
PHC Mouthpiece System - Paxman/Halstead/Chidell system of 12 rims + 3 finishes + 15 cups => 540 possible combinations. "An imaginative player will select cups to musically enhance particular areas of the horn's repertoire, range, and tone quality, without any unsettling effect on the embouchure."
YouTube video on how a [Taylor trumpet] mouthpiece is made. Irritating accompanying music, but turn the sound off and it's an interesting tour of mouthpiece-making.
Thompson Edition Mouthpieces - a series of mouthpieces designed by David Thompson, solo horn of the Barcelona Symphony; 3 models. Also offered is a unique embouchure visualizer, with a rim identical to the Thompson mouthpieces.
Warburton - mouthpiece specialist
Houser Horn Mouthpieces - used by players in the Philadelphia Orchestra, Metropolitan Opera, etc. Stainless steel rims available; the site has a page to answer the question “Why stainless steel?” Newsletter available.
Curry Horn Mouthpieces - precision mouthpieces available in 3 diameters with 2 cup shapes. Custom and delrin-rim mouthpieces available.
Marcinkiewicz Inc. - Pro-line Concert Hall horn mouthpieces
Mouthpiece Express - offers horn mouthpieces by Bach, Curry, DEG, Denis Wick, Kelly, Marcinkiewicz, Schilke, Stork, and Yamaha. Also: custom plating and engraving; a plethora of accessories: adapters, BERPs, balancers, Buzz Buddies, guards, care kits, cases, stands, lubricants, metronomes & tuners, mutes & mute bags, and more.
Greg Black - Five basic models of horn mouthpiece, plus their BP model, said to be "the only horn mouthpiece designed for players who wear braces."
whf -mouthpieces - Austrian maker of mouthpieces for the Vienna horn.
Mouthpiece Bore Sizes
A chart by Osmun.com correlating bore sizes: decimal with fraction, number, or letter.
Mouthpiece Specification Chart (all brasses) - by Mouthpiece Express
Select up to four (short list of standard mouthpiece makers - but no Holton, no Moosewood, etc.) and see a chart comparing their specifications.