Windows/Mac/Linux (Firefox): Firefox extension Readeroo integrates with your Del.icio.us bookmarks to make keeping up with your backlog of unread links a cinch. Let's say you stumble onto a link on Lifehacker that you're dying to read, but—weirdly enough—you're at work and can't get to it (damn boss looking over your shoulder). Rather than tossing it carelessly into your ever-expanding, unmanageable pile of unread bookmarks, add it to Del.icio.us by clicking your Readeroo Add button—which automatically bookmarks the link in Del.icio.us with a "toread" tag. When you finally get a spare minute or two, just click Readeroo's Read button to cycle through your unread links and automatically tag the item "donereading" (the actual tags used are customisable if you don't like the defaults).
If you still prefer adding and tagging your Del.icio.us bookmarks using a more robust extension like Del.icio.us Complete, you can still do so—just tag the item with "toread" and Readeroo will pick up on it. You'll need to provide the extension with your Del.icio.us username and password the first time you use it. If you've got an unwieldy mess of Del.icio.us bookmarks you've never gotten around to reading, this extension is a must-have. Readeroo is a free download, works wherever Firefox does.