What are the largest social bookmarking sites that allow tagging (e.g., Delicious)?

http://kifi.com/?kcid=na-kp_social-quora-bmsearch is a service that will index the page for you as soon as you keep (or bookmark) it. You can add a note with tags upon bookmarking.  Also .... It's free Google Search Integration - It's integrated with Google Search (some other services make you pay for this features) Keyboard shortcut- to privately keep (cmmd +shift + option + K) Import your bookmarks - You can import bookmarks from your browser, Delicious, Pocket, Diigo, Evernote, Kippt, Instapaper, and http://pinboard.io/ Integrations - Sync your keeps with http://kifi.com/twitter and http://kifi.com/integrations/slack. Contextual Chat - Chat alongside any page or highlight the best part of the page and send it to a friend Work with your team - Option to keep something into a library visible only to your team members or publicly Discover vetted content - Discover your friends’ public bookmarks while searching Export - Take your keeps with you at any time Multi platform - Also available on iOS, Android, firefox, and the web. Check us out at http://kifi.com/?kcid=na-kp_social-quora-bmsearch

I use Contentle (http://www.contentle.com) - which enables to process bookmarks (with preview), images, videos, text snippets (GF-Markdown text). You can organize, annotate and then share it with whomever you want. Powerful search, different view modes, tagging and more. Contentle has RSS reader with ability to copy entry into bookmarks.

Reddit has something vaguely similar to tagging - subreddits - although it's more of a social news / interesting stuff type of site, and you have to submit links to subreddits one at a time, rather than tagging a link with lots of tags in one go. It's not really appropriate to submit stuff to a general reddit which would likely only be of interest to you personally, or a small group of people - you should create your own private reddit for that sort of thing.

IBM has an internal social bookmarking/tagging system called dogear that is apparently quite popular within IBM

Probably Google Bookmarks although I'm afraid that the vast majority of its users don't actually use tags.