Three suggestions with a Steven Wilson link (vaguely!)...
At the mellower end of the spectrum I'm surprised nobody has mentioned Blackfield, the side project of PT's Steven Wilson with Isreal's Aviv Geffen. Great self-titled album and a new one out in the coming months hopefully.
Just been listening to Mabool, the latest album from Orphaned Land. Not heard it all yet but sounds quite promising. Think Porcupine Tree and Opeth meeting for a jam in an Arabian desert. S.W. has been tipped to produce their next album.
And last (and most tenuously!!) Pure Reason Revolution who's "The Dark Third" is one of the, no, THE best album I've heard in ages. Apparrently SW was going to produce the album but couldn't fit them into his schedule. I'd say out of all theses suggestions PRR are probably the closest to PT in terms of contrasts between light and heavy and approach to music, but with very much their own sound.
Definately concurr with Opeth, and Riverside aren't too bad either. Couldn't get into any of ultralight's suggestions I'm afraid... Seem a bit out of place to me... more indie, except for Enter The Hunt... quite liked "One" (It's not a cover Metallica fans).
And just for a bit of vareity check out "Peeping Tom", the latest project from ex Faith No More frontman Mike Patton, my no.2 album of the year with tool's 10,000 days at no.3