At Wire, we've actually been working on the "autotagging" (& some more) for the past 2 years. We're focused on mobile and don't have a desktop version, yet. Our current version tags a page based on the keywords in metadata, title; we're still fine tuning it and will include the content in the future. The way Wire works is: instead of the conventional bookmarks, users just saves what they find and use the same search engine they use everyday to find it again. What the user saves is stored for offline viewing. In addition, we've also included an offline p2p feature that allows the user and an nearby offline friend to share what they've saved.

Check it out on: https://goo.gl/xMgxfJ

Pinboard.in doesn't "autotag" but it suggests tags and you can click them to have them added to the form you're saving. it's quick and easy.