No one has mentioned that, of course, much of the Baen stuff is available for free from the Baen free library. This means you can get:
Honor Harrington book one for free
The Drake stuff mentioned here.
Weber/Ringo have a series that starts with "March Upcountry"
Lois McMaster Bujolds series on Miles Vorkosigan (or something like that)
Ringo's posleen stuff
A bunch of Bolo stuff
Dalmas, Hogan, Moon, Pournelle, Turtledove, Taylor
Its really hard for me to pick out one -- except to say: how can anyone ask about military science fiction without seeing an answer that says, "Go check out Baen".
Start with the free library:
http://www.baen.com/library/