My team has been using Pganalyze (which has its own index advisor), and honestly, it's one of my favorite technical tools ever. I don't know how you could run a large-scale postgres setup without it. In addition to index advice, it also shows top queries, vacuums, etc, and monitors for a whole host of other issues out of the box.

They also have a ton of great content on their blog (5mins of postgres) where the founder will find blog posts by different authors/companies and analyze them in depth.