It's amazing how many official websites ABC runs for its hit show Lost (OK, maybe not THAT surprising). Here is a list of the ones I am aware of. (Beware, possible spoilers ahead!)
The official, official abc.com
site. Quite boring actually. As far as I know, nothing important there.
Oceanic-Air.com: this was the first "secret" site. Hacking this site will eventually lead you to...
Oceanic Flight 815.com: this is truly the official site. Episode index, cast & character info, etc, etc etc. And, of course, some hidden info as well.
The Hanso Foundation: look closely and you will find at least one "goodie."
Dharma Industries: I have yet to figure out how to hack this one...I have a few ideas, but they haven't worked yet. I'll keep you posted.
The Fuselage: official site for the crew of Lost, and it claims to be sponsored by J.J. Abrams. Has a cool message board.
The following two sites are of unknown veracity. Some say they are official, others disagree.
Drive Shaft's Official Site: it's unknown if this site if run by ABC, or if it's just a REALLY well done fan site.
The French Lady: all evidence points to this being UNofficial, but who knows. Either way, it's kinda cool.
And last, but not least, are a couple interesting unofficial links...
Lost-Media.com: the best fan site I have found.
MAJOR SPOILER ALERT!!: This is not a fan site (though it was discovered by the fans); it is a link to info found on Google. It may shed light on the infamous numbers, it may not, but I find it too much of a coincidence to ignore. Click
here.