For somewhat similar vocals try
Chevelle - Point #1
For similarly progressive metal music with a lot of emphasis on the bass
Isis - Oceanic / Panopticon / Wavering Radiant
Enslaved - Ruun / Vertebrae
And definitely get into Porcupine Tree though they're much more mellow. Good songs to start with for a Tool fan would probably be Russia On Ice (one of their best of all time), Strip the Soul, or Anesthetize.
And while Opeth is nothing like them, they are one of those bands that no one really sounds like (well, successfully, there's a lot of clone attempts like Farmakon or Gwynbleidd) and a lot of Tool fans get into them. Start with Blackwater Park and Damnation and go backwards if you are more into the metal side and forwards if you like the prog side.