I'm going to make some really nonstandard "prog" reccomendations. Because I'm like that.
Amorphis - Tales From The Thousand Lakes - Fuck Opeth, this stuf is tr00 mixing of prog and death metal. If you don't have this album already... well you probably should.
King Crimson - Discipline - A King Crimson album on this list really requires no explanation. Besides the ones that have been noted already in this thread, this is definitely their best.
Tool - Aenema - I know Tool aren't really popular on this board, and you may already have this and like/dislike it. But I'm going to say it anyway. It's a great album, as well as other Tool albums, if you choose to hear it that way. It took me a long time to get into Tool because of this, but now they're one of my favorite bands.
X Japan - Art Of Life - The album is only 29 minutes long or so, but it's all one big song with an intensely long piano section. It's prog-metal-tastic and I would say its something like a proggier, faster, version of SSOASS Maiden (of course not as amazing). Maybe just download the song or something, since material by X Japan is pretty impossible to find though.
Yes - 90125 - This album has some of my personal favorites by Yes. It's one of the few albums made that successfully bridges pop and prog. Definitely worth owning.