I keep them too - I always re-read and also lend out to family and friends. I have STACKS of books, I work in a bookshop and spend more than I earm most of the time . We have so many books in the house - DH hates it because he says bookshelves look 'messy'.
If you don't want to keep them, I love the idea of donating to nursing homes etc. Otherwise you can always do the 'pay it forward' thing - leave a book on a park bench or train seat with a little note saying "Hope you enjoy this as much as I did!"