How on earth have we gotten this far into Sci-Fi recommendations without mentioning Andre Norton?
Norton specialized in blending science and fantasy. Her Witch World series starts out as sci-fi/adventure, and she's written a very interesting series about time traveling (Time Traders). Here's the first book free and DRM-free from Baen:
http://www.baenebooks.com/p-429-time-traders.aspx
and the second book free from Gutenberg:
http://www.gutenberg.org/ebooks/19651
There's also a whole bunch of other excellent free sci-fi of hers from Gutenberg.
http://www.gutenberg.org/ebooks/author/7021
She also wrote a series about humanity dealing with a galactic empire by becoming mercenaries. Here's the first 2 books free and DRM-free from Baen:
http://www.baenebooks.com/p-396-star-soldiers.aspx