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BRIAN OBERKIRCH, CREATIVE COMMONS ATTRIBUTION-SHARE ALIKE GENERIC 2.0

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September unique visitors: 987,190
Change from August: +12.4%
Funding: $750,000 from Spark Capital and Union Square Ventures

Why it's unique: Tumblr posts are quick and concise like tweets, but they make use of photos, videos, quotations, conversations, and other mixed media. "Twitter is this application for letting people know what you're doing; Tumblr is the place where you share those experiences," explains founder David Karp. It's also renowned for elegant templates, easy-to-use publishing tools, and ever-customizable fonts, backgrounds, and layouts.

How it plans to create revenue: Karp says Tumblr has 500,000 registered users, a number that's growing nearly 20% every month. In November, the site will start offering more ways to post content for a monthly or annual fee; all of the site's current features will remain free. The site's backers say it's in a league of its own: "It's a tumblogging service, which is fundamentally different than a microblogging service," says Fred Wilson (pictured), partner of Union Square Ventures, which has invested in Twitter and Tumblr.