I have recently collected some beech wood for my tanks. When you are collecting wood you need to be sure that no chemicals (weed killers etc.) have been sprayed on it (this can be the case in some parks), and the wood isn't near a road. I know that beech is safe, but I'm not sure about others, although I think beech looks great anyway. I prefer to use live wood, that has been blown/snapped off the trees in bad weather (don't vandalise). In winter it is fine to use live wood, as long as you prepare it properly. I prefer to use live wood, as it is soooo much easier to debark; the bark comes off in strips, but with dead wood you have to hack at it for ages. Also, if you collect wood from trees and not ponds/waterways, then there won't be any aquatic hitch hikers. when you have debarked and scrubbed the wood, soak it in water for a few weeks (the longer the better) changing the water daily, to remove any sap etc. You can speed up this process by boiling the wood occasionally. When it has been soaked for weeks, give it a good clean, and put it in the tank.
BTW, don't use any chemicals at any point, as bleach etc. will kill your fish.
cheers