Asimov's Foundation Trilogy would be a very cool series, and Peter Jackson is probably the one to do it.

Ender's on its way: http://www.frescopictures.com/movies/ender/endersgame_update.html (though you'll notice this update is a year old...)

The Otherland series by Tad Williams
The Gunslinger by Stephen King

Dan Simmons' Hyperion, definitely

Ditto on the Foundation trilogy.

Second on:

Ringworld Series
Ender Series
Snow Crash

Agree about Ringworld even though I've never taken the time to read it. If they ever did it I'm guessing there will be some young Xbox fans wondering "why is the Halo movie named Ringworld?"

I vote for:
Belgariad by David Eddings
and obviously:
Eye of the World by Robert Jordan

I think I've heard rumors that they are actually considering Eye of the World.

Hitchhiker's Guide to the Galaxy would be on the top of my list but I think it's already got the green light. I haven't heard when it's planned to release.

Neuromancer - William Gibson
Farewell Horizontal - K.W. Jeter
Iron Man - Marvel Comics
Tesla, Man Out of Time - Cheney
The Difference Engine: Charles Babbage and the Quest to Build the First Computer - Swede

+1 on Ender and Neuromancer
also on Farewell Horizontal. That was a great book, and would make a fabulous movie. I thought I was the only one who ever read that one. :)
Oryx and Crake

+1 on Ender's Game
+1 on Snow Crash

I also think that Cryptonomicon by Neal Stephenson would have some potential as a techno-thriller.

And I thought I was the only one who's read Farewell Horizontal !
Just imagine what modern FX could do with the bike grippers ! ! !

+1 on Ender's Game & Snow Crash. Note: a friend of mine was working on the Snow Crash computer game a while back (he worked for Pyrotechnics) but it was scrapped.

I would add (several of these have already been mentioned):
Eye of the World (Jordan) - though it would be tough to compete with LOTR and the genre might be too close
Cryptonomicon - this has just the right mix of thriller, sci-fi, and historical fiction
Hyperion - this would make an *amazing* movie
Any of Salvatore's books on Drizzt Do'Urden - could have some of the same issues as Eye of the World, but I would love to see a movie based in the Underdark.
Talisman - King & Straub, though it looks like it might be in the works - http://horror.about.com/b/a/034984.htm

I second the nominations for Drizzt Do'Urden based movies and Wheel of Time adaptations. And the movie go'ers thought LoTR was long! Hehe. Seriously though just think of Winters Heart being made into a movie, with all the channeling at the end. It would be an orgy of CGI.

Foundation series would make a cool movie but it'd probably be a little too deep and political for alot of people if they made it true to the books.

Best for adaptation would have to be the Sandman series by Neil Gaiman.

+1 for Foundation, Hyperion, Snow Crash, Neuromancer and Ender.
I would add Dan Simmon's latest, Ilium: it would require many special effects but would make a great film.
See http://www.amazon.com/exec/obidos/tg/detail/-/0380978938/102-0513442-6056113?v=glance

I'm not so sure about "technical geek" movies, but I'm holding out for the day when people get enough sense to make "The Secret People" into a movie. And yes, I'm aware that my personal goddess Audrey Hepburn was already in an obscure movie of the same name. I'm talking about the book by John Wyndham.

Grass by Shery Tepper
Belwether by Conie Willis
Any Terry Prachette book
Any Ian M Banks book
and of course the Game of Thrones series.