I hope you know Baen - the publisher that specializes in that kind of stuff.
They have quite a few boks - usually first part of the series and suchlike - available on the net for free.
http://www.baen.com/library/
They also distribute some of their books on CDs and those CDs are legally available here
http://baencd.thefifthimperium.com/
Be careful!
Some of those series are quite addictive when you like hard and/or military SF