Closet full of clothes or lined with rockwool works great.
Simple broadband absorption doesn't require anything fancy in terms of design, materials, or construction. You just need to have dense absorption that is deep enough (6"~2 feet, the thicker the better). Absorption in corners is important, not just in the middle of the walls.
So if all you want is a dead space to record vocals in, your primary concerns are budget and appearance. If you don't care what it looks like you can build a hut out of old mattresses or bales of hay or rolls of pink fiberglass. If you want a clean, professional appearance you can buy some slick commercial products like Auralex or get creative with rockwool and fabric and lumber.
Oh, and put a rug on the floor.
If, on the other hand, you need a space that is acoustically decoupled (soundproof) from the adjoining space where you may have a drumkit or a dishwasher or sleeping kids, then that's a whole nother can of worms that is addressed completely seperately.
Cheers.
<message edited by yep on January 08, 07 1:00 PM>