Building a raised bed or rebuilding a raised bed? This little post from Umbra over at Grist offers insight into which materials to consider.
Natural materials are the best bet, although they might be a little more expensive and perhaps a little harder to find. Reusing has obvious benefits as well, but is not without some cons. Be sure you know which materials are safe and what the reused materials are made from. Pressure treated lumber for vegetables may be a bit controversial.
A good basic plan for building the raised bed can also make all the difference, too. This video offers some interesting ideas and techniques, plus his dogs are pretty entertaining. Garden Girl offers a good video on building a raised bed and a ton of information on urban sustainable living. Inspiring stuff!