The two I started and never finished are William Gibson's Pattern Recognition and Eric Brown's Helix.

Pattern Recognition I picked up on a recommendation, Helix had a cool-sounding blurb on the back. I started them, got a little ways in, and lost interest. But that typically doesn't happen with books. Normally, I'll get a book, and slog my way through, no matter how long it takes. I typically don't give up halfway (or less) through.

Now, I do have a lot of books I've bought and for some reason, never started. Usually those are books I've picked up at garage sales, or larger purchases from the bookstore. I'll put those on my "to read" list, and every time I finish one book, another one I want to read more will force its way to the top. So I have a lot of books that are just lying on my shelves unread.