With the exception of Amazon, who evidently weren't interested in cooperating, Tivo does a great job of this. You get some Amazon content but it's not the good stuff. For Hulu and Netflix you get unified search and you can do things like come across an in-progress movie in the guide, see that it's available on Netflix, and start watching it right away. I really wish Amazon would let Tivo have a decent app with Prime support.
Edit: Sure enough, as soon as I posted this I came across an article saying that Tivo will be getting Amazon Prime this fall, as well as Vudu. http://zatznotfunny.com/2014-08/amazon-instant-vudu-coming-t...