Hallucinogen has been mentioned - good. Simon's work is the place to start.
Juno Reactor has been mentioned, though they really haven't been trance since Beyond the Infinite. Still, they're mandatory listening if you like it dark and mind-bending.
If you're not afraid of tracking down some hard-to-find releases, look into older Astral Projection material, back from when they were good, and older Infected Mushroom, back from when they were dark and mind-bending. AP's Trust in Trance is absolutely essential and every track on it is golden. Production is a bit dated now, but the sheer raw energy and intensity have never been paralelled to date. IM's Classical Mushroom is likewise mandatory listening, though it's not as heavy or dance-oritented as some of the latter works. These albums are a piece of trance history now and need to be in your collection.
Other stuff you need:
Cosmosis - Cosmology, Contact. Bilbo Baggins makes some really impressive dark spacy stuff. Intelligent, progressive psy-trance from a classically-trained master.
Doof - It's About Time. This disk is a more minimal, more trance-oriented version of early-day Shpongle, and is mandatory in any trance-head's collection. This is some really, really tripped-out and completely out there music.
Raja Ram's Stash Bag #2 - I'm not a fan of compilations, but this set is amazing. Pure infectious energy and rampant psychedelia. [Edit: for a first purchase, this is probably your best bet. If you like the set, you can start tracking down the individual artists based on what tracks you like.]
There's really too much to mention. You'll probably get a kick out of Dark Soho and Total Eclipse in addition to the above mentioned stuff.
