read later

 Read later  is an indication on (or relating to) a website that one wants to save the URL to come back and read the content at a future time.

Rick Mendes
Rick Mendes uses hashtags #readlater and #readinglist on his Known site 1. He provides additional details and discussion here.

Chris Aldrich
has been experimenting with general reading posts/workflows, some of which are discussed here: A New Reading Post-type for Bookmarking and Reading Workflow. In December 2016, I began using PressForward to internally save private bookmarks for things I want to read later, as well as a way to easily publish them as read posts when done.

gRegor Morrill
has been experimenting by posting book titles prefixed with "Want to read:" since 2016-04-06
 * https://gregorlove.com/2016/04/want-to-read-a-kim/
 * Using  and   for the book title. The identifier's href is in the format.
 * Manually POSSE to Goodreads.

Silos

 * Goodreads allows one to mark books as "Want to Read"
 * Pocket - acquired by Mozilla on 2/17/17
 * Instapaper - announced 8/23/16 that Pinterest is buying the service and shutting down a portion of their development
 * ReadKit
 * Flipboard
 * Safari Reading List, built natively into the browser
 * Facebook Save
 * Send to Kindle
 * Reading.am Though this service is philosophically meant to indicate what you're actively reading right now (rather than saving for later), its functionality is roughly similar to others in this category.

Defunct Silos

 * ReadingPack (defunct - see site-deaths)
 * Readability (bookmarking service shut down 9/30/16 - see site-deaths)

Archiving Apps with similar functionality

 * Evernote
 * OneNote
 * Historious
 * KeepEverything
 * Diigo
 * Wallabag opensource, self-hosting

bookmark sites
These allow one to bookmark URLs which could potentially be tagged as read/unread, thereby providing this type of read it later functionality.
 * Delicious
 * Pinboard (or their sub-app Paperback
 * Pinterest
 * Readwise is adding read later functionality in late 2021
 * Micro.blog can save bookmarks via Micropub. For Micro.blog Premium subscribers, Micro.blog supports highlighting on bookmarked web pages and saves an archive of the page and linked resources.