Antimatter.

[Kermit]

Would an object made of antimatter look exactly like its matter counterpart and just have a different electric charge, or would it appear different and have different properties?

[[Tycho?]]

It would be the same I believe. Anti-hydrogen is the only anti-atom that has yet been made, as far as I know it works just like hydrogen only its anti-matter.

 

http://en.wikipedia.org/wiki/Anti-hydrogen

[Severian]

It would have very small differences because the electroweak interaction does not conserve CP, but you probably wouldn't notice.