[see also mutes and cases]
Gazley Horn Pencil Clip (Amazon.com)
Also available from Osmun and Musicians Friend
Don't be without one! As John Barrows once said, the sign of a professional is always having a pencil with you at every lesson, practice session, and rehearsal! Buy a dozen or two and give them to your friends and students!
Comfortable! They also protect the metal. Cleverly constructed to give you a choice of two heights, $6 for a set of 3 at Ken Pope.
Leather Valve Lever Protectors by Alexander
Horn string for rotary valves - available from Woodwind-Brasswind
The Gig Bag was designed for horn players to handily carry all the stuff we have to have. Cordura fabric, hand and (removable) shoulder carrying straps.
Leather Specialities Company makes excellent leather hand guards for most (not tuba) brass instruments. Alas, no photos of what you're getting (the photo here is from Ken Pope, who offers them); but I've seen a sample and they're very nice. There are 99 - count 'em! - 99 different models for horn, each made for a specific make and model (e.g. "Conn 8D (East Lake) French Horn Hand Guard - Leather - Black $19.78"). The site is young; perhaps photos will follow later. They also make various bags and pouches.
From the web site: “A horn support device and leather guard in one. More ergonomic than the pinkie ring or flipper. Fits both Kruspe and Geyer wrap horns. Used by the Cleveland and Boston Symphony Orchestras. The Clebsch Strap comes with the leather strap and a metal screw mounting plate. The mounting plate should be installed by a brass repair technician.” $45 + $5 S+H. See the article on it by John Ericson.
Neotech Wrap
Alexander Hand Strap
$60 from Ken Pope.
BERP= Buzz Extension and Resistance Piece. Enables mouthpiece buzzing with the mouthpiece in position on the instrument. An indispensable practice aid for any brass player. $20.
The JoyKey, just out, looks like a revolutionary accessory that all brass instruments will want to have: it drains water from tubing automatically, using a high-tech replaceable metal filter. It “maintains the integrity of the air column of the instrument” while continuously draining water. In the demo video, strategically placed JoyKey metal knobs drip excess moisture:
P.E.T.E. - Personal Embouchure Training Exerciser. “Placing the disk behind the lips and pulling the exerciser forward prompts an isometric response from the muscles of the lips, preparing them for the forces encountered in brass playing.”
Mouthpiece pressure adapter - "Teaches low-pressure playing. A valve inside the device opens when too much pressure is used, allowing the air to escape before it reaches the instrument. Adjustable."Mouthpiece Pressure Training Device - available from Thompson Editions, made by Gebr. Alexander. Fits between mouthpiece and leadpipe; lets air escape if maximum desired pressure is exceeded.
Valve Casing Weights
Ayscue Horn Stand
. Made from wood and poly tubing, folds up.
Hercules horn stand.ChopSaver Lip Balm
[Left to Right] Mouthpiece Case (from Osmun) Horn String (DeHaro) Mouthpiece Brush (DeHaro)
Stand shelf for horn players. Made by horn player, educator, and craftsman Dave Donaldson. Features shelf for stuff, knobs for mutes, pencil holes in front.
Wooden Music Stand Clip - indispensable if you ever play outdoors to hold sheet music down when a breeze comes up!
Musicmaide makes Plexiglas stand clips:
Woodwind/Brasswind - Good price on the Konig and Meyer Horn Stand ($20) - they call it a "multi-purpose instrument stand", but it suits the horn much better than what they list as a 'French horn stand.' Actually, many guitar stands work just as well at about half the price, such as this one offered by Musician's Friend.
The folks at Embouchures.com offer a number of unusual but useful accessories, including brass buzz pipes (a.k.a. "Fish Callers"), a "great practice tool for staying in shape when on vacation" or when you don't have your horn.
Tony Halstead sells something similar - his "Halstead Buzz-Buddy":
Medlin Horns offer the Just-a-Pipe buzz pipe
OcToBrass - “a new compact buzzing and practicing instrument for the musician who must spend time away from the horn.” Equipped with a valve to regulate pressure to match the natural resistance of the horn. 
Offered by Chasons Music (Dave Chamberlain), which has other products as well, including the Acoustic Personal Monitor, which enables players to hear yourself more clearly with one ear while you hear the band/ensemble with the other ear - description and photos at the Chasons web site.
Etymotic - makes Musicians Earplugs™ that reduce sound levels 20 dB without distorting speech and music. Get some. I said, “Get Some!”. Before it’s too late.
Braceguard - "Lip protection for musicians and all brace patients."
Horn Stuff - accessories (BERP, mic, tuner, metronome, pencil clip), bags, books, cases (gig/flight bags, and a terrific padded detachable bell case with wheels!), horns (Atkinson, Hoyer), hand guards, horn stands (they offer their own slick fold-up compact horn stand, as well as the K&M horn stand described above (but cheaper)), music, mutes (Kowalchuk, Dennis Wick, Tom Crown, Silent Brass), triple horns (Hoyer); repair shop.
MusiChem - "Better Music Through Chemistry". Specialists in valve oils and grease for brass instruments. I haven't tried them, but the horn lists have had some very positive testimonials about their products lately.
Good: Mighty Bright XtraFlex Super LED stand light, $15. Better: Duet LED Music light 
$25. Best: (brightest) of all is their Orchestra Light - 14-20 hours of battery life (3 AA batteries) $75. 9 bright white LEDs, 2 brightness levels. Several more models not pictured here - check them out at the site.Lampcraft makes the Giglight and the Super Giglight - top of the line clip-on music stand lighting. Powered by 4 AA batteries.
ProTec - makes high-quality horn cases, gig bags, mouthpiece pouches, and various care & maintenance products.
Sorley's Horn Shop - offers repair services, Marcus Bonna cases, flippers and finger hooks, and their own GS1 Geyer model horn.
Adjustable Hand Rest. aka "Duck's Foot." Takes the weight off the little finger.
Amazon now lists musical instruments and accessories for sale, usually as an agent for other music stores. If you have an account with Amazon, it is usually easier to order the item from Amazon than the original seller. Pencil clips, for example.
Anderson Music Stand - the most elegant and beautiful fold-up music stand ever (no photo here, but go to the site and see the demo videos). Sculpted from one solid piece of hardwood. Comes with its own carrying case. Lifetime warranty. Woods: brazilian cherry, hard maple, natural walnut, ebonized walnut. Optional: Lotus [stand] Light - elegant, attachable, with Lithium batteries.
Fantasy Music Stands
Beautiful, hand-made wooden music stands of all types by UK craftsman John Tyler.
Mister Standman Music Stands (made in Idaho)
•75 designs available in 10 different woods
and many different stain colors.•
Amazon.com also lists quite a number of wooden music stands...
Center Pitch Clip On Tuner for brass
Visualizer for hornErgoHorn
Heavy horn? Back problems? Looking very much like Pip Eastop's Pipstick, the ErgoHorn stick makes it possible to play a 'weightless' horn and thus reduce real or potential back, arm, and neck problems.
Horn to Marching Mellophone Adapter (L - from Osmun; R - from wwbw.com)
Made in Austria, marketed by Denis Wick in the UK. Battery-powered lip massager using "your own mouthpiece". Like one of those vibrating massage chairs - but for the lips! Read more about it here (pdf) and here (pdf).
Katahdin Studio Furniture of Lewiston, Maine, also makes one-of-a-kind handcrafted music stands, as well as lovely sheet music cabinets:
Horn Harmonics - A 'wheel' that shows alternate fingerings and harmonics series numbers 'at a glance.'
Thompson Edition
Thompson Edition is one of the more attractive web sites around with high quality items for sale to match, including cases for horns and mutes, mouthpieces, and sheet music (50,000 titles). Free worldwide shipping (with a flat handling fee of $3.95).
Dillon Music - music manuscript paper, books, CDs, method books, solos & duets, video/DVDs, finger buttons, gig bags & cases, hand guards, instrument stands, metronomes, mouthpiece pouches, mutes, oils & lubricants, tuners, valve caps, used stuff.
Rayburn Music - instruments, accessories
Encore Music - huge array of every kind of oil, grease, swabs & brushes, other Care & Maintenance accessories, CDs, horn ties, pins, mugs, and much more.
TuneUp Systems - textbook and CD to train the ear.