Index basically bookmarks stuff, using tags. Evernote does that but also clips whole pages or simplified versions, stores docs, allows rich text, works offline, and lots more. EN has its drawbacks but is overall far more productive. Using Index as a layer on top, as its makers more sensibly suggest when they're not tweeting "Sorry Evernote", would add complexity and require online access all the time.
Flexibly nestable tags
It's promoted as an Evernote replacement but doesn't come close.