As far I can see in the Clam thread, no online clam wallet are available, you can use exchange sites or sites that offer Clam services like Just-Dice but that is not very secure, specially with exchanges sites, Just-Dice is fine because is a service that have been running without issues for more than 3 years, also most all the Clams in circulation are there, but is usually because people want to stake and invest there.

 If you're looking to have control of your Clams locally you should consider the official Clam Client that is on the Clam thread, you will need to take a time to sync it, but after that you will have control of your wallet better than using a 3rd party. Also you need to take measures to protect your clams in your computer like encrypting your wallet and stuff like that. Anyways if someone knows about a trusted online Clam wallet option besides Just-dice they will soon or later inform us here.