This has every Luigi's Mansion site you could think of and information on the sites. I haven't found any just on Luigi's Mansion, so if you know ones I can put on, e-mail me.

Official Sites
Nintendo At E3 Has info, lots of screenshots for Luigi's Mansion.
Official Luigi's Mansion Site Kind of weird. It has some information and a couple of downloads.

Other Luigi's Mansion Sites
There aren't any I could find. Please tell me one. Just e-mail me at theboss@gamecubestation.20m.com