Most people would put a game or a video or message board chock full of excellent advice here, wouldn't they? Well. I've established I'm not normal or 'most people' by now, right?
The site that exports my books reviews, image of the book and rating and all, is Goodreads (yes, that links to my profile so you can add me if you want), which recently rolled out some changes. Based on site metrics and where other users rate and shelve certain books, you can get automated recommendations. I've gone through the automatic rating system and found a few books I'd meant to put on my to-read list. But, far more useful is where you can give and receive recommendation requests. There are a lot of new requests on any given day, so anything you post is likely to scroll well out of sight within a week or two, though it'll stay at the top for your friends.
Much more useful is that I can go through the requests and rack my brain for books that fulfill people's requests. I love sharing books I loved with other people, and regularly whine (privately) that not enough people I talk to on a regular basis read the writers I like.
You can comment on why you recommended a book, or what part of the request stood out to you when you go through the process. Then people can "like" your recommendation, or comment back.
Out of all of the books I've recommended, 95% of the users have added them to their to-read lists (the others, the users had already read), and five people have added me to follow my reviews. It's quite an ego boost.
It wouldn't be as addictive without the feedback, but I'm betting I'd still sink more time than I should into browsing the requests to find someone looking for speculative fiction or dark humor, which are the requests I gravitate toward. I've recommended a lot of Christoper Moore's books in the humor category, and Dan Wells' I Am Not A Serial Killer has gone out to more than one member of the Goodreads community. Peter S. Beagle's books keep coming up when people ask about classic fantasy, and I've found a lot of excuses to promote Seanan McGuire and her Toby Daye books, as well as the Newsflesh series she writes as Mira Grant.
Are there any books that you try to find excuses to recommend to people? Who are your favorite authors who don't have nearly as many fans as you think they should?