blockchain.info very popular easy to use and very secure. and now blockchain.info have released the second version and better again
Yes it is popular but talking about the security, I am not sure of it. I am an avid user of blockchain.info wallet before but recently I got some bugs using it, it was resolved though. But I am afraid to encounter any bug again and lose my coins so I decided to import my private key to electrum and now I am happy with a software based one. I would suggest you avoid using web based ones like blockhain.info for more security