I figure it's kind of dumb to recommend albums everyone knows, so here goes:
Grandaddy's The Sophtware Slump is a very good but unfortunately very unknown album. Think something like OK Computer plus Twin Peaks. The nine minute opening track is a favorite of mine.
Caribou's Andorra is also good; think modern-day psychedalia.
The Cardigans' First Band On The Moon, which features their hit song, Lovefool, is overlooked as a full album. It's one of those ones that leaps from track to track without a pause in between. On top of that, the mastering is incredibly good.
Mad Pudding's Grand Hotel is an album I haven't stopped listening to since I saw them live in 1998. It's the most Canadian album ever, but it's practically impossible to find.
Supergrass's Road to Rouen is one of my top five albums, even though it only has nine songs. It's melancholy British guitar pop, sort of hard to describe. Think cloudgazing on a rainy day.
Jon Brion's Punch-Drunk Love soundtrack, another top five. I could listen to this all day long and not get tired of it.
Ugly Casanova's Sharpen Your Teeth, an offshoot of Modest Mouse, is a dusty, smoky album, with lots of weird stuff in it. If you like Modest Mouse, chances are you'll like this.
The Dandy Warhols' Thirteen Tales From Urban Bohemia. It's difficult to describe this one.
The Flaming Lips' Zaireeka is the most experimental album ever: there are four CDs, each one with the same songs on it, only each corresponding track has something different on it. You have to get four CD players to play it, and it's worth it.