ok but which one of them are widely used and require no or minimal registration?
You are possibly overlooking the question you should be asking.
Which ones can be trusted to hold your funds?
And the answer to that is, the ones whose identity is known, the ones who make use of cold storage -- preferably with the private keys protected using something like SSSS (Shamir's secret sharing scheme) stored in a bank vault, etc.
That narrows the list pretty significantly -- Coinlab/Mt.Gox (U.S.--soon), Bitcoin-Central/Paymium (France), ??
Now there are others that might qualify, such as Coinbase (U.S.), but they aren't shared wallets -- addresses in your wallet are not shared with other customers of that service. And Blockchain.info/wallet is a hybrid EWallet and addresses are not shared.
Also, two-factor authentication is an important criteria for an EWallet:
Which Two-factor authentication methods are available at which exchanges?
- http://bitcoin.stackexchange.com/questions/4113
