One site I subscribe to is BookBub. I'd say its basically the same as eBookStage, another site I subscribe to. Both are free for readers to use.
I get daily emails from both alerting me of deals in my favourite genres, and while both usually show me the same deals, sometimes one will have a listing which the other does not. This makes subscribing to both worthwhile.
Deals generally cover all eBook options from Kindle to Kobo to Apple iBooks. One difference is that for the author 'follow' option, there is a larger selection on BookBub than eBookStage.
A newer site similar to these has been brought to my attention. Book Swag is also free for readers to subscribe to, and lists discounts and deals on various books. It's rather new and there does not appear to be many listed deals as of yet, but as time goes on, I'm sure it'll grow and prove to be just as useful for the reader as BookBub and eBookStage. A bonus, for myself personally, is that I love the aesthetics of the site itself. I can't speak to the quality of the actual service, but I'm going to give it a try!
What sites or services do you use to keep track of the latest book deals and discounts? Let me know in the comments.