Too many favourites already mentioned but I can't believe 'Dark Side of the Moon' hasn't been mentioned yet!

I much prefer it over 'The Wall' if only because it is about being insane (I guess 'Wish You Were Here' should be mentioned too).
'Deadwing' is growing on me but 'In Absentia' definitely deserves a mention.
I would also mention 'Selling England by the Pound' which is all about the decaying of values in English Culture, and 'The Lamb Lies Down on Broadway' both by Genesis.
If you have a look at the following list (
http://en.wikipedia.org/wiki/List_of_concept_albums), I would include numerous albums such as 'Dirt' by Alice In Chains, 'Ride The Lightning' and 'Master of Puppets', 'Once Was Not' by Cryptopsy, and 'Undertow', 'Aenima' and 'Lateralus' by Tool although to be fair I had never really thought of any of these albums as being concept albums before ...