You should check out all the SF books available for free from Baen. You can get them all here:
http://baencd.thefifthimperium.com/
I'd personally recommend the Honor Harrington series, although I enjoyed many other books from Baen. If you're into alternative history at all you should check out the 1632 series.
You really can't go wrong browsing this stuff and randomly picking something to read. If you don't like it, it was free and you can delete it. And if you do like it, go buy your copy over at webscriptions for the unbelievably reasonable price of $6 with no DRM.
-Marcy