What is generally thought of as 'classic Sci-Fi.' It can be less plot-driven, as it explores in-depth the science that makes the story possible. If you prefer your space opera plausible and well-reserched, give these a try:
Isaac Asimov (If ever a book was better than the movie, it was I, Robot.)
Poul Anderson
Greg Bear
David Brin
James Hogan
Larry Niven
Frederik Pohl
Fred Saberhagen
Carl Sagan
Melissa Scott
Clifford Simak
A.E. Van Vogt
Thanks to Andy for these
$7.19
ISBN-13: 9780553294385
Availability: Usually Ships in 1-5 days
Published: Spectra,
11/01/1991 $6.92
ISBN-13: 9780671578589
Availability: Not Currently Available
Published: Baen,
02/01/2000 $7.19
ISBN-13: 9780345435248
Availability: Usually Ships in 1-5 days
Published: Ballantine Books,
07/01/2000 $7.19
ISBN-13: 9780553278743
Availability: Usually Ships in 1-5 days
Published: Spectra,
11/01/1997 $25.95
ISBN-13: 9780595166213
Availability: Usually Ships in 1-5 days
Published: Backinprint.com,
12/01/2000 $7.91
ISBN-13: 9781416537250
Availability: Not Currently Available
Published: Baen,
08/01/2007 $7.99
ISBN-13: 9780765357830
Availability: Not Currently Available
Published: Tor Science Fiction,
08/01/2008 $13.46
ISBN-13: 9780345475831
Availability: Usually Ships in 1-5 days
Published: Del Rey,
10/01/2004 $6.92
ISBN-13: 9780765350596
Availability: Not Currently Available
Published: Tor Science Fiction,
05/01/2007 $15.30
ISBN-13: 9780345409461
Availability: Usually Ships in 1-5 days
Published: Ballantine Books,
02/01/1997 $13.46
ISBN-13: 9780765320773
Availability: Usually Ships in 1-5 days
Published: Orb Books,
07/01/2008 $7.95
ISBN-13: 9781606643693
Availability: Usually Ships in 1-5 days
Published: Aegypan,
02/01/2009 
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