After reading Ringworld as a kid, Halo was a bit of a let down. For all those who grew up with Halo, I wonder if Ringworld stands up. I'd add the new Young Miles omnibus by Lois McMaster Bujold. Remnant Population by Elizabeth Moon. Hexwood by Diana Wynne Jones (read it again recently and the quantum computer is interesting). Moon of Three Rings and Sargasso of Space by Andre Norton.