BlockChain.info is a utility site offering a number of free ad-supported services:
Alternative block chain browser
Live updating list of unconfirmed transactions
Data and charts providing insight into the Bitcoin P2P network
Double spend tracking
JSON and block explorer compatible api
Multi-signature coordination service (for use with M-Of-N transactions)
The site's availability status and service history is monitored using PingDom.
The site was launched on August 30, 2011, originally as the PiUK-BlockChain-Browser. It is maintained by Ben Reeves and was built with the intended purpose of recording double spend attacks and various currency statistics.
On March 26, 2012 the Blockchain for Android mobile app was released. On April 20, 2012 the Blockchain ios app for the Apple iPhone, iPod touch and iPad was released globally on the App Store.