1) Old Man's War by John Scalzi -- Sort of like "Starship Troopers", except you get eternal youth, not citizenship, and the volunteers are all senior citizens.
2) Red Thunder by John Varley -- "Rocket Ship Galileo", only they're going to the Moon, instead of Mars. And the
3) Survival by Julie E. Czerneda -- A scientist just wants to be left alone to study genetic diversity in salmon, but is drawn into a galactic mystery as entire planets are stripped bare of all organic life.
I'm not sure it's a coincidence that two of the three are obvious Heinlein homages (to be fair, there's more sex in "Red Thunder" than "Galileo", but the analogies are obvious). Still, they're all fantastic. Heck, I'll even add in another one:
4) Kevin J. Anderson's Saga of the Seven Suns. Definitely fluffy space opera, but Anderson has an extremely sneaky ending to the first book that, while not totally unexpected (to me; a friend was pleasantly surprised), was nicely evil.