Made from a flight feather from the wing of a good-sized bird, the quill was the standard writing implement from the Dark Ages up until the middle of the 19th century. Feathers from different birds had different writing properties; swan feathers were considered the best, goose feathers the most common, and crow feathers were suited for the drawing of fine, straight lines. In colonial America, Thomas Jefferson went so far as to breed his own geese so he would never run out of writing instruments.
In the mid-19th century, pen nibs made of steel became popular and began to replace the traditional quill; fountain pens, ballpoints and other modern pens became the preferred writing instruments. For those who enjoy the romanticism of writing with a quill, modern varieties are still available online, or at many book or hobby stores. While some are still cut from real feathers, many are synthetic and fitted with a steel nib. Modern quills are also often used for calligraphy, and ballpoint versions are often sold with guestbooks for weddings.
A number of companies and artists are now making replicas of historic quills, as well as ones related to movies and pop culture. The Liberty Bell Museum's gift shop, for example, sells a variety of carved feather quills and modern ballpoint versions. Movie replica shops, such as Heavenly Swords and Replica Power, sell a series of Harry Potter-themed quills.
There are a number of resources online that teach you how to make your own quill pen. Medieval Writing has detailed descriptions of how quills were made by medieval scribes, and on Flick.com, Liralen Li provides extensive step-by-step instructions, complete with photos and commentary.