I have to second the Friedman recommendation. Her Coldfire trilogy is like nothing else I've ever read, and is fantastic. I hesitate to describe it much, I think that knowing too much might spoil some of the fun of reading it.
As a follow-up on my earlier post:
Tad Williams - he's best known for the Memory, Sorrow and Thorn trilogy, which he is following up with another series. It's fairly standard (but very well done) young-man-on-epic-quest fantasy. I didn't enjoy his Otherland series as much but that's science fiction anyway.
Melanie Rawn - The Dragon Prince trilogy, and it's follow-up Dragon Star trilogy. Lots of small-scale politics rather than the epic scale of some of the current series. Stay away from her Exiles "trilogy" - due to some health issues she has not written the last book in the past 15 years.
I'll add one more with a little hesitation. I don't see Katherine Kurtz discussed much in fantasy conversations, but her Deryni series is one of the longest running series around (started in 1970, she just published book #16). She's got a degree in medieval history, and her world is so close to ours that it may as well be an alternate history "if there were a psychically/magically gifted race". That can be either good or bad. My hesitation in recommending is that I find the first trilogy a little tedious, but it needs to be read to understand the other books, which are among my favorites.