I still go in to book stores and libraries and browse--don't do much buying or checking out books any more, but I do get ideas this way.
One online site I like to "browse" for books on is
www.fantasticfiction.com . As the name implies, this only works for fiction books. It gives an alphabetic listing of authors--you can click on an author's name to see all their books. You can also narrow your search to specific fiction genres. I find the site very valuable for figuring out which book is the next book in a series I'm reading.