These are some of the fish which I've found to be "discus compatible"
Keep in mind that your tank should be a discus tank first - make sure the
other fish you intend to keep with discus conform to discus conditions,
and not vice-versa! Always quarantine any fish for at least two to four
weeks (preferably longer) before mixing them in with discus.
In addition to the fish pictured below, I've kept the following successfully with discus:

Gold Nugget Plecotomus - Baryancistrus sp.
Upside downcatfish - Synodontis nigriventris
Polka dot catfish - Synodontis angelicus
Black neon tetra - Hyphessobrycon herbertaxlerodi
American flag fish - Jordanella floridae
Twig catfish - Farlowella acus
Whiptail catifish - Rineloricaria sp.
Kuhli loach - Pangio (Acanthophthalmus) kuhli
Dwarf Suckermouth Catfish - Ottocinclus affinis
(more information on the catfish can be found on www.planetcatfish.com)

 
Cardinal Tetra Paracheirodon Axelrodi - These brilliantly colored fish do well with discus. Cardinals can live in the warmer, acidic water that discus prefer and a school of Cardinals can "draw" out a shy discus. Cardinal Tetras are related to the Neon Tetra, but will grow slightly longer (up to 2" or about 5cm) and are hardier at warmer temperatures. Try to keep at least nine in your tank. Just remember the rule of companion fish size - "if it fits inside the discus' mouth, it will end up in there". For more information, see here.
 
Clown Loach Botia Macracantha - These  generally do well with discus and are good to keep as bottom feeders. Try to keep at least two in your tank. Although these fish can grow large in nature (up to 12" or 30cm), usually they don't exceed 6" (15cm) in a home tank.   I've found that if these loaches are a lot larger than the discus, these loaches can scare the discus during feeding (the loaches sometimes go into a frenzy). For more information, see Loaches Online.
Corydoras Catfish - These small (usually no larger than 3" or 7.5cm) catfish are excellent bottom feeders, very peaceful, and like to be in groups. There are several different varities of these and some will tolerate the temperatures of up to 30ºC or 86ºF. Two points to remember - these catfish cannot tolerate temperature higher than 90 degrees F (about 33 degrees C) and sometimes these fish carry flukes and hexamita. I've had good success keeping the Corydoras Panda with my discus. For more information, see Planet Catfish.
Imperial Zebra Plecostomus Hypancistrus zebra - I've had good luck keeping this catfish with my discus.  Unlike most other plecostomus, the zebra is more of a carnivore. There are several different varieties of plecostomus, some stay small, others will outgrow small tanks. Some will develop a "taste" for the mucus on the discus' skin, so be watchful. Alternatives to the Plecostomus, would be a Farlowella Farlowella species., Whiptail catfish Rineloricaria sp., the Dwarf Suckermouth catfish Ottocinclus sp., or the Bushynose/ Bristlenose catfish, Ancistrus sp. For more information, see Planet Catfish.
 
Siamese Algea Eater, Crossocheilus siamensis - This fish is a very good algea eater and scavenger.  I've rarely seen them cause any trouble with discus.  Sometimes these fish are a bit skittish and that can frighten a discus, but I've only seen a couple SAEs actually harass discus.  The only fault with keeping this fish with discus is that they would rather eat the discus food instead of 'working' for algea! For more information, see Algae Eating Cyprinids.