Well, as mentioned before, Arthur C. Clarke - (Rendezvous with Rama, Fountains of paradise) Isaac Asimov - I enjoyed "The Gods themselfes.." just a few months ago. Also the Robot-Series should match your criteria. The Foundation-Serie is also awesome, but does not realy match. I further enjoyed Robert A. Heinlein's "The Moon Is a Harsh Mistress" not too long ago. I liked it, but the style of it is realy needs getting used to. Also, "The Forever War" by Haldemann - does contain a handfull of scifi-gadgets but in return you get some realy hard scifi