Originally an American maker of writing instruments, Parker is now based in the United Kingdom. Parker is one of the better-known pen companies, and created some of the best known fountain pens.

The company got its start in 1888, by George Parker. Parker was frustrated with the fountain pens of the day. Though they were somewhat of an improvement over dip pens of the day, they did tend to leak, be messy, and were temperamental. Parker's innovation was they "Lucky Curve." In the feed was a small curved tube. This prevent a blob of ink from spilling out on the paper.

In 1921, the Duofold was born. Until the Duofold, pens were mostly made from black hard rubber. The Duofold first introduced a red-orange hard rubber. These pens are known as "Big Reds." It was a hit.

Parker followed the Duofold with the Vacumatic. The Vacumatic was innovative in that it provided a filling mechanism that allowed the complete diameter of the barrel be used to store ink.

In 1941, Parker introduced what would be regarded as the best fountain pen ever produced: the Parker 51. The 51 had very modern styling (even by today's standards), and very reliable. This pen was produced into the 1970s. They were so popular that some would by just the cap, to give the appearance they had the 51.

Today, Parker is still manufacturing fountain pens, as well as other writing instruments. They updated and reintroduced the Duofold in the late 1980s, and more recently have done the same with the Parker 51.

For collectors, Parker is very fun to collect. Over their hundred-plus-year history, they have produced a wide variety of pens, with each era having a very distinct look.

Sources:
http://www.pendemonium.com
http://www.ettnet.se/~boris/ An excellent Parker reference