World Music Supply, the "Musical Warehouse to the World", is an online store for guitars, drums, studio gear, and other musical instruments and equipment.
Woodwind & Brasswind is an online store of musical instruments with a focus on woodwind and brasswind instruments. They sell largely to schools in addition to professionals.
Guitar Center is the largest retail music gear company in the world, known for selling guitars as well as many other musical instruments and accessories.
Musician's Friend is an online store that sells musical instruments and accessories. They are known for price matching, free shipping, and a good return policy.
zZounds is an online music store that sells a huge variety of musical instrument gear. They specialize in low prices and low shipping costs.