*I've read through this, so if I repeat what others have said, I apologise now.*
-Dan Simmons. People have mentioned Hyperion series (possibly my favourite series of all time), but his later series Illium is also pretty amazing. Imagine Homer's Illiad, but the gods are humans who just have superior technology... And there are robots... and alternate realities... (Hard to describe Simmons' plots)
-Roger Zelazny. The Amber Series is just amazing in terms of imagination, characters and the scope of it all. A guy wakes up, finds out he's a member of a royal family who can create worlds at their whim... Can't say much more.
Most of Zelzany is pretty awesome, actually. To name the standouts would be Wizard World (two books in one, Changeling and Madwand), Lord of Light and Jack of Shadows.
- Clive Barker. Mainly known for horror (and he's good at that), Weaveworld and his Imajica books are both incredible fantasy books. A world preserved in the threads of a carpet? Come on...
His Abarat series are also AMAZING, but only really if you have the hardcover copy with his paintings in it. Story of a girl who travels to a place (with a guy whose 7 brothers live in the antlers on his head) with 25 islands, each representing an hour of the day. At the island of Midnight lives the Carrion family, who want to wage war on the islands of the day... And yes, there is a 25th hour...
- Harry Harrison. Stainless Steel Rat! While not that heavy on the "science" part of the sci-fi, it is an awesome character driven series about a distant future where interstellar travel is possible, most worlds are law abiding... Except for Slippery Jim diGriz, a professional thief, con etc... Who never kills, which I think is a great restriction to have on a character. (Ok, yes, once or twice he kills someone, but he never WANTS to kill anyone...)
-Fritz Leiber. Fafhrd and the Gray Mouser series. A big barbarian from the north with a big sword, a little guy, once apprentice to a wizard, with a little sword, adventuring...
- Robert E Howard. Has anyone mentioned Conan yet? I'm not a Conan-purist like some, but if you DO want to read the good Conan the Barbarian books, focus on the ones written JUST by Robert E Howard.
On the subject of old sword and sorcery...
- L. Sprague DeCamp and Fletcher Pratt. Together they wrote a series called the Magical Misadventures of Harold Shea... First book being The Incompleat Enchanter. The adventures of a guy who has figured out the mathematics of magic, so to speak, and with the right formula can transport himself to other worlds (first one he visits is a world based around Norse mythology). Bit of an antithesis to the idea of Conan, big burly guy solving problems with muscles, Harold Shea tries to solve things with logic, intellect... though he does have to fence to solve some problems.
-Julian May. Saga of the Exiles series is... well, initially set in the future, someone discovers a one-way gate to the Pliocene period, hoping to escape various things in the present. But when they get there, they find a race of human-like aliens have taken over the region, having been exiled themselves from their own galaxy.
Enjoy researching.
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