Docstoc is a document browsing and searching tool for "professional documents." Unlike Scribd, which has been casually called a "YouTube for documents," Docstoc is going for a more business-oriented crowd. At least that's their pitch. The features are very similar, with a communal sense of document sharing and live-viewer for MS Office documents, PDF files, and other popular file types.
One thing that does set it apart is its user profiles and document request system. Each user gets their own profile, complete with as much personal information as their willing to share, along with a link to their LinkedIn profile (if they have one). Underneath that is a full listing of their documents, which can be searched along with everyone else's shared content.
The document request system is a little more interesting. It lets you request a document you're looking for by setting tags or keywords. This basically sets up an alert that will keep an eye on other Docstoc user submissions. When someone uploads a document that matches your criteria, you'll receive an alert. Whether or not it really matches what you were looking for is anyone's guess, but the idea is that there's hope instead of dead ends when it comes to tracking down files.
The service demoed at today's TechCrunch40 conference, although is still in private beta. To get access, you can sign up on their front page. There's also a 6-month old pre-beta walkthrough on YouTube, which I've embedded after the break.