Website discovery engine StumbleUpon had 500 million hits last month and its traffic has doubled in 6 months, the company announced today.

StumbleUpon lets users jump through websites by adding a navigation toolbar to the top of every website visited. Users can vote up the site and comment on it, and then immediately “stumble” to the next website without too much trouble. Registered users can pick a number of topics they are interested in and StumbleUpon delivers content that fits those themes whenever they jump to the next website.

Like news aggregators, StumbleUpon it an attempt to solve discovery problems. There is quite a bit of content on the Internet — so much so that the number of available web addresses has officially run out ahead of a revamp that will increase the number of available web addresses. Since there is so much noise, users flock to sites like news aggregator Reddit and StumbleUpon to curate their news and content and help users find just what they are looking for.

StumbleUpon’s announcement comes fresh on the heels of Reddit’s recent traffic announcement that the news aggregator had more than 1 billion hits in January. A number of other startups, like app discovery tool Appitalism, have popped up to try to tackle the problem of content and application discovery as well.

StumbleUpon also recently launched an Android application designed to solve the same discovery problems that applications have. There are already more than 300,000 apps on the iPhone App Store, and more than 1,000 more appear daily. There are also more than 115,000 apps on the Android Marketplace, according to So even on application stores there is a lot of noise — and users need some tools to curate their content and help point out new applications.

The company was launched out of Canada in 2002 and didn’t move to the San Francisco Bay Area until 2006. It was acquired by eBay a year later for $75 million — though it has since been taken private. The San Francisco, Calif.-based company raised $1.5 million from Google’s founding investor, Ram Shriram, Lotus founder Mitch Kapor, Topic founder Ariel Poler, and angel investor Ron Conway.