Ok lets start with the basics, I hope this will be a main focal point for adding new elements to Powder Toy with Simon's cooperation.
First off:
Hydrogen: a fairly useful tool, it is combustible only in the presence of oxygen, it should either under certain circumstances:
Form Dihydrogen Monoxide vs Hydroxide: The only way to form either one of these without forming much of the other is through 1 of the 2 options: to form water, more energy (in the form of temperature) is required to form the bonds and 2) to form Hydroxide one must have a lower energy level than what is required to make water, thus the 3rd bond won't adhere and thus you have HO (hydroxide)
Hydrogen when bombarded with Neutrons can for Deuterium (Heavy hydrogen/ Deuterium oxide: Heavy Water) and Tritium (Radioactive isotope of Hydrogen)
Carbon: Another very important tool, you can burn it in the form of Coal, and you can forget about Graphite, I don't really see it being implemented as being a useful form of Carbon. The Last form of Carbon is Diamond, you can make diamond under a massive amount of Heat and Pressure, and I don't mean a little, I mean just shy of the pressure that a Nuclear Reaction creates. Carbon when bonded with Oxygen can form Carbon Dioxide and Carbon Monoxide, but Carbon Monoxide isn't important right now; Carbon Dioxide can be used as a flame retardant, it can be cooled down to a liquid state through either cooling it or pressurizing it, this is what you find in common house hold fire extinguishers. Carbon can be bonded with Hydrogen and Oxygen to form sugars, we don't need every form of Sugars out there, probably just a simple sugar of glucose or something. Sugars can be used to promote plant growth and form crystalline structures. Carbon and Hydrogen can also be used to form Hydrocarbons, I suggest to form many of the other more complex hydrocarbons you will require the use of more energy, the more energy, the more complex the hydrocarbon and its bonds (ie: Methane, Ethane, Butane, Propane... I suggest not implementing other types of bonds such as Alkenes or Alkynes as that will become too complex for the endeavor). With Carbon bonded with Oxygen in Hydrocarbons you should simply invest in just Ethers. Hydrocarbons can be used to make Oil or other fuels in the game. When an OH (hydroxide) is mixed in a hydrocarbon, you generally get an Alcohol, alcohols are important in many different ways, they can be used as a fuel like Ethanol (2 carbons with an OH).
Nitrogen: Another important diatomic element, Nitrogen can be used in a gaseous form can be used as a flame retardant, when bonded with Oxygen you can get your Nitrous you find in the game already, when bonded with Hydrogen you can get Ammonium and Ammonia, which are common house hold Bases that can be used to counter the acid. I don't know how Nitrogen acts with Carbon though. Nitrogen can be used with hydrogen in Complex Hydrocarbons to form Amides, Amines and Esters.
Chlorine: not as important as the others but it is useful for the creation of Hydrocarbons with a halide in them to form Halocarbons (Hydrocarbons with a group 17 halogen in it)
Silicon: Used in many electrical applications, this Metaloid is used in all complex electrical devices. It is found in sand (sand is Silicon's oxidation)
Copper, Iron, Tin, and Lead: some of the most common metals in our sociological world, Copper is a very good conductor, just shy of Silver and Gold. Copper is used in sculptures. It is also mixed (not bonded) with Tin to form Bronze (not brass: brass is copper with Zinc). Copper can be cooled down enough to form Super conductors. Iron can be used as a magnet of devices, when an electrical current is run through it, it makes an electromagnet, Iron's rust can be used for the creation of Thermite. With the mixing of some Carbon, Iron, and a bit of Magnesium, you get the all mighty Steel. Tin is a very viable heat insulator, it reflects heat incredibly and is used in oil fires by personal because it reflects so much heat, it is used in the manufacturing of tin foil and tin cans. Lead is used for many applications including in Nuclear Reactors where it can be used. Lead is very resistant to Acid because isn't very reactive.
How about an element that conducts instantaneously (like the conductive wall)?
Or alternatively, to keep it real, an element that conducts electricity REALLY fast. I've done some research and the best I could come up with is Silver (already mentioned).
Maybe there's something else?
Simon, maybe make a sticky for all elements and wall suggestions.