StumbleUpon has been rumored to be on the market for the last half year or so, and now TechCrunch is reporting eBay may have agreed to purchase the social bookmark/website discovery startup for $40-45 million.

StumbleUpon is a rapidly growing service with more than 2 million users already.

According to TechCrunch, Google, eBay, and AOL had all expressed interest in buying StumbleUpon.

The company provides users with a browser toolbar that lets them rate web sites they visit with a thumbs up or a thumbs down. They can also write reviews of each site. Perhaps the most distinctive feature is the "Stumble!" button that takes viewers to a website rated highly by users with similar interests. It's sort of like channel flipping on a television and only finding programs that your friends have suggested you might like.