There's nothing new about online bookmarking. What makes Instapaper so amazing is not what it has... but rather what it doesn't have.
If you've dabbled with tools like Delicious and Google Bookmarks, then you know that there is no lack of online (and free) tools to help you organize your online content and share it with others. Instapaper has been around for a while, but what makes it that much more powerful is how well it now integrates with an e-reader and the iPhone. That's the real win here (but even if you don't have a digital reader or an iPhone, it is still worth checking out).
Read it later.
Instapaper has a very basic sign-up process that then allows you to place a bookmarklet that says "Read Later" in your browser bar. Any time you come across a piece of content online that you don't have time to read at that moment, you can save it for later without ever leaving the page or clicking/typing anything additional. Simple and basic, and that's the point. On top of that, you can then download everything you are saving into PDF, Kindle and ePub format - essentially creating your own magazine of worthwhile content (say goodbye to physically printing all of those interesting articles and Blog posts). Instapaper also formats the content in a highly readable and visually appealing way.
Though there is a free iPhone application for Instapaper, the full version (at $4.99) is worth every tiny penny (you can compare both versions here: Instapaper iPhone App). With the app, you can read all of your content, archive it, set up specific folders and everything else that you can do with the online version. On top of that, you can actually save content to read later directly from your iPhone (it's a bit of a workaround, but the instructions are very easy and clear to follow).
It's basic, it's simple, and it's easy (yes, it bears repeating).
Unlike other online bookmarking tools that enable you to share (like an online social network) or that have multiple fields to rename the title, add tags, etc... Instapaper keeps it simple (one click and it's saved) and that's what makes it so powerful. Coupling that to the e-reader and the iPhone app make the circle complete. If you like to read and consume information online, this is the grand slam homerun of content management.
Do yourself a favour and get Instapaper.