Plenty of awesome recommendations here so far. I have a TON: (repeats for emphasis)
Opeth, Katatonia (especially Discouraged Ones and later), Amorphis, Amon Amarth, Ayreon, Gojira, Arcturus (The Sham Mirrors), Borknagar, Born Of Osiris, Dark Tranquillity, Darkest Hour, Daylight Dies, Fields of the Nephilim (not necessarily metal but very good), Hypocrisy, Kamelot, Lake of Tears (Forever Autumn), Meshuggah, My Dying Bride (amazing band, check out A Line of Deathless Kings), November's Doom, Novembre, Swallow the Sun (another amazing band, highly recommended for Hope and The Morning Never Came), Symphony X (Russell Allen!), Tesseract (One), Therion (another of my highly recommended, if you don't like Gothic Kabbalah you are insane), Textures, Tool (again not really metal, more prog rock but amazing), assorted Trivium, Veil of Maya for hardcore, Witchery (Opeth's drummer), While Heaven Wept (Vast Oceans Lachrymose), Unearth for metalcore (Darkness in the Light is awesome).
Female vocalists and/or blended:
After Forever, Amaranthe (Elize mmmm), Epica, Nightwish (Century Child), Tristania (Illumination).
Long block of text but hopefully it helps get some of the lesser known good bands (IMO anyway) out there recognized. Hoping to pick up some good ones here as well. Always looking for good tunes.