There are just too many!! AGH. Characins come in EVERY shape and size imaginable. Very popular tankmates for Discus, most are peaceful. The larger South American Characins such as Pirhanas and Pacus shouldn't be kept with Discus. Below are a few choices of Characins to be kept with Discus.
Black Neon Tetras - Popular fish, similar to the Cardinal Tetra, just this one's black. Grows to 1.5"
Bleeding Heart Tetras - Very popular fish with Discus, colorful and peaceful. Grows to 1.5".
Bloodfin Tetras - Not very popular, but very cool looking. Peaceful. Grows to 2".
Cardinal Tetras - THE most popular tankmate for Discus. Very very colorful, and are peaceful and easy to keep. Grows to 2", and is best kept in a school of 5 or more.
Emperor Tetras- Very cool and colorful fish, not very popular, or available for that matter. Grows to about 1.5".
Lemon Tetras - One of the most popular tankmates for Discus. Colorful, and peaceful, grow to 2".
Pencilfish - Very popular tankmate for Discus, very peaceful, colorful, and stay small, 1". Nice looking when kept in a group.
Penguin Tetras - Very popular tankmate, look more like the Black Neons, but have a bigger tail, peaceful, and grow to about 1.5".
Rasboras - Very colorful, may be kept, but I wouldnt reccomend it. If they are kept in a group they will bite fins.
Red Eye Tetras - Cool looking fish, weird eyes, peaceful, and grow to about 2".
Silvertip Tetras - Cool fish, neat looking fins, and are peaceful in groups under 4, very active, grow to about 1.5".