There are lots of sites with free aircraft available at a variety of
sites. Most require free membership, some require paid, and some give
you better download speeds etc if you do pay. Some also have news and
message forums. Some of the most popular sites:

www.avsim.com
www.simviation.com
www.fs2004.com
www.suclaro.com
www.simouthouse.com

There are many more.

There are also a lot of commercial vendors who sell addon aircraft and
scenery. I won't list these but you'll find them on Not too long ago
there was a survey on the AVSIM forums regarding the quality of the
aircraft..