GOOD MATERIAL= Foam sound panel
INEXPENSIVE PRODUCT= Thick heavy window drapes, floor to ceiling length. You can open/close to adjust the rooms reverb to taste for each project.
I've had very good luck with window drapes. As Mutt said, you can open and close to adjust the treatment. And if you cna find the right ones they can look very good.
Rummage sales, Goodwill, and other types of "thrift shops" can be a good source.
I'm planning on building some acoustic panels out of fiber glass and wood frames for my vocal booths. Here are some good instructions: http://www.geocities.com/jonrisch/a1.htm